(function(global, factory) {
  /*jshint -W030 */
  'use strict';
  typeof exports === 'object' && typeof module !== 'undefined'
    ? factory(exports)
    : typeof define === 'function' && define.amd
    ? define(['exports'], factory)
    : global.async
    ? factory((global.neo_async = global.neo_async || {}))
    : factory((global.async = global.async || {}));
})(this, function(exports) {
  'use strict';

  var noop = function noop() {};
  var throwError = function throwError() {
    throw new Error('Callback was already called.');
  };

  var DEFAULT_TIMES = 5;
  var DEFAULT_INTERVAL = 0;

  var obj = 'object';
  var func = 'function';
  var isArray = Array.isArray;
  var nativeKeys = Object.keys;
  var nativePush = Array.prototype.push;
  var iteratorSymbol = typeof Symbol === func && Symbol.iterator;

  var nextTick, asyncNextTick, asyncSetImmediate;
  createImmediate();

  /**
   * @memberof async
   * @namespace each
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.each(array, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done();
   *   }, num * 10);
   * };
   * async.each(array, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [[1, 0], [2, 2], [3, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.each(object, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done();
   *   }, num * 10);
   * };
   * async.each(object, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b']]
   * });
   *
   * @example
   *
   * // break
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num !== 2);
   *   }, num * 10);
   * };
   * async.each(array, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 2]
   * });
   *
   */
  var each = createEach(arrayEach, baseEach, symbolEach);

  /**
   * @memberof async
   * @namespace map
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.map(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2];
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.map(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [[1, 0], [2, 2], [3, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.map(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.map(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b']]
   * });
   *
   */
  var map = createMap(arrayEachIndex, baseEachIndex, symbolEachIndex, true);

  /**
   * @memberof async
   * @namespace mapValues
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValues(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3, '2': 2 }
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValues(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3, '2': 2 }
   *   console.log(order); // [[1, 0], [2, 2], [3, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValues(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3, c: 2 }
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValues(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3, c: 2 }
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b']]
   * });
   *
   */
  var mapValues = createMap(arrayEachIndex, baseEachKey, symbolEachKey, false);

  /**
   * @memberof async
   * @namespace filter
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filter(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3];
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filter(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3];
   *   console.log(order); // [[1, 0], [2, 2], [3, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filter(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3];
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filter(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3];
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b']]
   * });
   *
   */
  var filter = createFilter(arrayEachIndexValue, baseEachIndexValue, symbolEachIndexValue, true);

  /**
   * @memberof async
   * @namespace filterSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterSeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3];
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterSeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3]
   *   console.log(order); // [[1, 0], [3, 1], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterSeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3]
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterSeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3]
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c']]
   * });
   *
   */
  var filterSeries = createFilterSeries(true);

  /**
   * @memberof async
   * @namespace filterLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3]
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.filterLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3]
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  var filterLimit = createFilterLimit(true);

  /**
   * @memberof async
   * @namespace reject
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.reject(array, iterator, function(err, res) {
   *   console.log(res); // [2];
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.reject(array, iterator, function(err, res) {
   *   console.log(res); // [2];
   *   console.log(order); // [[1, 0], [2, 2], [3, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.reject(object, iterator, function(err, res) {
   *   console.log(res); // [2];
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.reject(object, iterator, function(err, res) {
   *   console.log(res); // [2];
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b']]
   * });
   *
   */
  var reject = createFilter(arrayEachIndexValue, baseEachIndexValue, symbolEachIndexValue, false);

  /**
   * @memberof async
   * @namespace rejectSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.rejectSeries(array, iterator, function(err, res) {
   *   console.log(res); // [2];
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.rejectSeries(object, iterator, function(err, res) {
   *   console.log(res); // [2];
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.rejectSeries(object, iterator, function(err, res) {
   *   console.log(res); // [2];
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c']]
   * });
   *
   */
  var rejectSeries = createFilterSeries(false);

  /**
   * @memberof async
   * @namespace rejectLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.rejectLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [4, 2]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.rejectLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [4, 2]
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.rejectLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [4, 2]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.rejectLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [4, 2]
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  var rejectLimit = createFilterLimit(false);

  /**
   * @memberof async
   * @namespace detect
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detect(array, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detect(array, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [[1, 0]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detect(object, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detect(object, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [[1, 'a']]
   * });
   *
   */
  var detect = createDetect(arrayEachValue, baseEachValue, symbolEachValue, true);

  /**
   * @memberof async
   * @namespace detectSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectSeries(array, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectSeries(array, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [[1, 0]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectSeries(object, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectSeries(object, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [[1, 'a']]
   * });
   *
   */
  var detectSeries = createDetectSeries(true);

  /**
   * @memberof async
   * @namespace detectLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [[1, 0]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.detectLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // 1
   *   console.log(order); // [[1, 'a']]
   * });
   *
   */
  var detectLimit = createDetectLimit(true);

  /**
   * @memberof async
   * @namespace every
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.every(array, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [1, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.every(array, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [[1, 0], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.every(object, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [1, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.every(object, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [[1, 'a'], [2, 'c']]
   * });
   *
   */
  var every = createEvery(arrayEachValue, baseEachValue, symbolEachValue);

  /**
   * @memberof async
   * @namespace everySeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everySeries(array, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everySeries(array, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [[1, 0], [3, 1], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everySeries(object, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everySeries(object, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [[1, 'a'], [3, 'b'] [2, 'c']]
   * });
   *
   */
  var everySeries = createEverySeries();

  /**
   * @memberof async
   * @namespace everyLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everyLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [1, 3, 5, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everyLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everyLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [1, 3, 5, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.everyLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // false
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e']]
   * });
   *
   */
  var everyLimit = createEveryLimit();

  /**
   * @memberof async
   * @namespace pick
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pick(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3 }
   *   console.log(order); // [1, 2, 3, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pick(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3 }
   *   console.log(order); // [[0, 1], [2, 2], [3, 1], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pick(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3 }
   *   console.log(order); // [1, 2, 3, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pick(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3 }
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b'], [4, 'd']]
   * });
   *
   */
  var pick = createPick(arrayEachIndexValue, baseEachKeyValue, symbolEachKeyValue, true);

  /**
   * @memberof async
   * @namespace pickSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickSeries(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3 }
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickSeries(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3 }
   *   console.log(order); // [[0, 1], [3, 1], [2, 2], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickSeries(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3 }
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickSeries(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3 }
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c'], [4, 'd']]
   * });
   *
   */
  var pickSeries = createPickSeries(true);

  /**
   * @memberof async
   * @namespace pickLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 5, '2': 3 }
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 5, '2': 3 }
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 5, c: 3 }
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.pickLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 5, c: 3 }
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  var pickLimit = createPickLimit(true);

  /**
   * @memberof async
   * @namespace omit
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omit(array, iterator, function(err, res) {
   *   console.log(res); // { '2': 2, '3': 4 }
   *   console.log(order); // [1, 2, 3, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omit(array, iterator, function(err, res) {
   *   console.log(res); // { '2': 2, '3': 4 }
   *   console.log(order); // [[0, 1], [2, 2], [3, 1], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omit(object, iterator, function(err, res) {
   *   console.log(res); // { c: 2, d: 4 }
   *   console.log(order); // [1, 2, 3, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omit(object, iterator, function(err, res) {
   *   console.log(res); // { c: 2, d: 4 }
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b'], [4, 'd']]
   * });
   *
   */
  var omit = createPick(arrayEachIndexValue, baseEachKeyValue, symbolEachKeyValue, false);

  /**
   * @memberof async
   * @namespace omitSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitSeries(array, iterator, function(err, res) {
   *   console.log(res); // { '2': 2, '3': 4 }
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2, 4];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitSeries(array, iterator, function(err, res) {
   *   console.log(res); // { '2': 2, '3': 4 }
   *   console.log(order); // [[0, 1], [3, 1], [2, 2], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitSeries(object, iterator, function(err, res) {
   *   console.log(res); // { c: 2, d: 4 }
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitSeries(object, iterator, function(err, res) {
   *   console.log(res); // { c: 2, d: 4 }
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c'], [4, 'd']]
   * });
   *
   */
  var omitSeries = createPickSeries(false);

  /**
   * @memberof async
   * @namespace omitLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '3': 4, '4': 2 }
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '3': 4, '4': 2 }
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { d: 4, e: 2 }
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.omitLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { d: 4, e: 2 }
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  var omitLimit = createPickLimit(false);

  /**
   * @memberof async
   * @namespace transform
   * @param {Array|Object} collection
   * @param {Array|Object|Function} [accumulator]
   * @param {Function} [iterator]
   * @param {Function} [callback]
   * @example
   *
   * // array
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     result.push(num)
   *     done();
   *   }, num * 10);
   * };
   * async.transform(collection, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4]
   *   console.log(order); // [1, 2, 3, 4]
   * });
   *
   * @example
   *
   * // array with index and accumulator
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     result[index] = num;
   *     done();
   *   }, num * 10);
   * };
   * async.transform(collection, {}, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3, '2': 2, '3': 4 }
   *   console.log(order); // [[1, 0], [2, 2], [3, 1], [4, 3]]
   * });
   *
   * @example
   *
   * // object with accumulator
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     result.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.transform(collection, [], iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4]
   *   console.log(order); // [1, 2, 3, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     result[key] = num;
   *     done();
   *   }, num * 10);
   * };
   * async.transform(collection, iterator, function(err, res) {
   *   console.log(res); //  { a: 1, b: 3, c: 2, d: 4 }
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b'], [4, 'd']]
   * });
   *
   */
  var transform = createTransform(arrayEachResult, baseEachResult, symbolEachResult);

  /**
   * @memberof async
   * @namespace sortBy
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBy(array, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3];
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBy(array, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [[1, 0], [2, 2], [3, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBy(object, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBy(object, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b']]
   * });
   *
   */
  var sortBy = createSortBy(arrayEachIndexValue, baseEachIndexValue, symbolEachIndexValue);

  /**
   * @memberof async
   * @namespace concat
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concat(array, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3];
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concat(array, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [[1, 0], [2, 2], [3, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concat(object, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [1, 2, 3]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concat(object, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [[1, 'a'], [2, 'c'], [3, 'b']]
   * });
   *
   */
  var concat = createConcat(arrayEachIndex, baseEachIndex, symbolEachIndex);

  /**
   * @memberof async
   * @namespace groupBy
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [4.2, 6.4, 6.1];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBy(array, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.1, 6.4] }
   *   console.log(order); // [4.2, 6.1, 6.4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [4.2, 6.4, 6.1];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBy(array, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.1, 6.4] }
   *   console.log(order); // [[4.2, 0], [6.1, 2], [6.4, 1]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 4.2, b: 6.4, c: 6.1 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBy(object, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.1, 6.4] }
   *   console.log(order); // [4.2, 6.1, 6.4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 4.2, b: 6.4, c: 6.1 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBy(object, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.1, 6.4] }
   *   console.log(order); // [[4.2, 'a'], [6.1, 'c'], [6.4, 'b']]
   * });
   *
   */
  var groupBy = createGroupBy(arrayEachValue, baseEachValue, symbolEachValue);

  /**
   * @memberof async
   * @namespace parallel
   * @param {Array|Object} tasks - functions
   * @param {Function} callback
   * @example
   *
   * var order = [];
   * var tasks = [
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(1);
   *      done(null, 1);
   *    }, 10);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(2);
   *      done(null, 2);
   *    }, 30);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(3);
   *      done(null, 3);
   *    }, 40);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(4);
   *      done(null, 4);
   *    }, 20);
   *  }
   * ];
   * async.parallel(tasks, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4];
   *   console.log(order); // [1, 4, 2, 3]
   * });
   *
   * @example
   *
   * var order = [];
   * var tasks = {
   *   'a': function(done) {
   *     setTimeout(function() {
   *       order.push(1);
   *       done(null, 1);
   *     }, 10);
   *   },
   *   'b': function(done) {
   *     setTimeout(function() {
   *       order.push(2);
   *       done(null, 2);
   *     }, 30);
   *   },
   *   'c': function(done) {
   *     setTimeout(function() {
   *       order.push(3);
   *       done(null, 3);
   *     }, 40);
   *   },
   *   'd': function(done) {
   *     setTimeout(function() {
   *       order.push(4);
   *       done(null, 4);
   *     }, 20);
   *   }
   * };
   * async.parallel(tasks, function(err, res) {
   *   console.log(res); // { a: 1, b: 2, c: 3, d:4 }
   *   console.log(order); // [1, 4, 2, 3]
   * });
   *
   */
  var parallel = createParallel(arrayEachFunc, baseEachFunc);

  /**
   * @memberof async
   * @namespace applyEach
   */
  var applyEach = createApplyEach(map);

  /**
   * @memberof async
   * @namespace applyEachSeries
   */
  var applyEachSeries = createApplyEach(mapSeries);

  /**
   * @memberof async
   * @namespace log
   */
  var log = createLogger('log');

  /**
   * @memberof async
   * @namespace dir
   */
  var dir = createLogger('dir');

  /**
   * @version 2.6.1
   * @namespace async
   */
  var index = {
    VERSION: '2.6.1',

    // Collections
    each: each,
    eachSeries: eachSeries,
    eachLimit: eachLimit,
    forEach: each,
    forEachSeries: eachSeries,
    forEachLimit: eachLimit,
    eachOf: each,
    eachOfSeries: eachSeries,
    eachOfLimit: eachLimit,
    forEachOf: each,
    forEachOfSeries: eachSeries,
    forEachOfLimit: eachLimit,
    map: map,
    mapSeries: mapSeries,
    mapLimit: mapLimit,
    mapValues: mapValues,
    mapValuesSeries: mapValuesSeries,
    mapValuesLimit: mapValuesLimit,
    filter: filter,
    filterSeries: filterSeries,
    filterLimit: filterLimit,
    select: filter,
    selectSeries: filterSeries,
    selectLimit: filterLimit,
    reject: reject,
    rejectSeries: rejectSeries,
    rejectLimit: rejectLimit,
    detect: detect,
    detectSeries: detectSeries,
    detectLimit: detectLimit,
    find: detect,
    findSeries: detectSeries,
    findLimit: detectLimit,
    pick: pick,
    pickSeries: pickSeries,
    pickLimit: pickLimit,
    omit: omit,
    omitSeries: omitSeries,
    omitLimit: omitLimit,
    reduce: reduce,
    inject: reduce,
    foldl: reduce,
    reduceRight: reduceRight,
    foldr: reduceRight,
    transform: transform,
    transformSeries: transformSeries,
    transformLimit: transformLimit,
    sortBy: sortBy,
    sortBySeries: sortBySeries,
    sortByLimit: sortByLimit,
    some: some,
    someSeries: someSeries,
    someLimit: someLimit,
    any: some,
    anySeries: someSeries,
    anyLimit: someLimit,
    every: every,
    everySeries: everySeries,
    everyLimit: everyLimit,
    all: every,
    allSeries: everySeries,
    allLimit: everyLimit,
    concat: concat,
    concatSeries: concatSeries,
    concatLimit: concatLimit,
    groupBy: groupBy,
    groupBySeries: groupBySeries,
    groupByLimit: groupByLimit,

    // Control Flow
    parallel: parallel,
    series: series,
    parallelLimit: parallelLimit,
    tryEach: tryEach,
    waterfall: waterfall,
    angelFall: angelFall,
    angelfall: angelFall,
    whilst: whilst,
    doWhilst: doWhilst,
    until: until,
    doUntil: doUntil,
    during: during,
    doDuring: doDuring,
    forever: forever,
    compose: compose,
    seq: seq,
    applyEach: applyEach,
    applyEachSeries: applyEachSeries,
    queue: queue,
    priorityQueue: priorityQueue,
    cargo: cargo,
    auto: auto,
    autoInject: autoInject,
    retry: retry,
    retryable: retryable,
    iterator: iterator,
    times: times,
    timesSeries: timesSeries,
    timesLimit: timesLimit,
    race: race,

    // Utils
    apply: apply,
    nextTick: asyncNextTick,
    setImmediate: asyncSetImmediate,
    memoize: memoize,
    unmemoize: unmemoize,
    ensureAsync: ensureAsync,
    constant: constant,
    asyncify: asyncify,
    wrapSync: asyncify,
    log: log,
    dir: dir,
    reflect: reflect,
    reflectAll: reflectAll,
    timeout: timeout,
    createLogger: createLogger,

    // Mode
    safe: safe,
    fast: fast
  };

  exports['default'] = index;
  baseEachSync(
    index,
    function(func, key) {
      exports[key] = func;
    },
    nativeKeys(index)
  );

  /**
   * @private
   */
  function createImmediate(safeMode) {
    var delay = function delay(fn) {
      var args = slice(arguments, 1);
      setTimeout(function() {
        fn.apply(null, args);
      });
    };
    asyncSetImmediate = typeof setImmediate === func ? setImmediate : delay;
    if (typeof process === obj && typeof process.nextTick === func) {
      nextTick = /^v0.10/.test(process.version) ? asyncSetImmediate : process.nextTick;
      asyncNextTick = /^v0/.test(process.version) ? asyncSetImmediate : process.nextTick;
    } else {
      asyncNextTick = nextTick = asyncSetImmediate;
    }
    if (safeMode === false) {
      nextTick = function(cb) {
        cb();
      };
    }
  }

  /* sync functions based on lodash */

  /**
   * Converts `arguments` to an array.
   *
   * @private
   * @param {Array} array = The array to slice.
   */
  function createArray(array) {
    var index = -1;
    var size = array.length;
    var result = Array(size);

    while (++index < size) {
      result[index] = array[index];
    }
    return result;
  }

  /**
   * Create an array from `start`
   *
   * @private
   * @param {Array} array - The array to slice.
   * @param {number} start - The start position.
   */
  function slice(array, start) {
    var end = array.length;
    var index = -1;
    var size = end - start;
    if (size <= 0) {
      return [];
    }
    var result = Array(size);

    while (++index < size) {
      result[index] = array[index + start];
    }
    return result;
  }

  /**
   * @private
   * @param {Object} object
   */
  function objectClone(object) {
    var keys = nativeKeys(object);
    var size = keys.length;
    var index = -1;
    var result = {};

    while (++index < size) {
      var key = keys[index];
      result[key] = object[key];
    }
    return result;
  }

  /**
   * Create an array with all falsey values removed.
   *
   * @private
   * @param {Array} array - The array to compact.
   */
  function compact(array) {
    var index = -1;
    var size = array.length;
    var result = [];

    while (++index < size) {
      var value = array[index];
      if (value) {
        result[result.length] = value;
      }
    }
    return result;
  }

  /**
   * Create an array of reverse sequence.
   *
   * @private
   * @param {Array} array - The array to reverse.
   */
  function reverse(array) {
    var index = -1;
    var size = array.length;
    var result = Array(size);
    var resIndex = size;

    while (++index < size) {
      result[--resIndex] = array[index];
    }
    return result;
  }

  /**
   * Checks if key exists in object property.
   *
   * @private
   * @param {Object} object - The object to inspect.
   * @param {string} key - The key to check.
   */
  function has(object, key) {
    return object.hasOwnProperty(key);
  }

  /**
   * Check if target exists in array.
   * @private
   * @param {Array} array
   * @param {*} target
   */
  function notInclude(array, target) {
    var index = -1;
    var size = array.length;

    while (++index < size) {
      if (array[index] === target) {
        return false;
      }
    }
    return true;
  }

  /**
   * @private
   * @param {Array} array - The array to iterate over.
   * @param {Function} iterator - The function invoked per iteration.
   */
  function arrayEachSync(array, iterator) {
    var index = -1;
    var size = array.length;

    while (++index < size) {
      iterator(array[index], index);
    }
    return array;
  }

  /**
   * @private
   * @param {Object} object - The object to iterate over.
   * @param {Function} iterator - The function invoked per iteration.
   * @param {Array} keys
   */
  function baseEachSync(object, iterator, keys) {
    var index = -1;
    var size = keys.length;

    while (++index < size) {
      var key = keys[index];
      iterator(object[key], key);
    }
    return object;
  }

  /**
   * @private
   * @param {number} n
   * @param {Function} iterator
   */
  function timesSync(n, iterator) {
    var index = -1;
    while (++index < n) {
      iterator(index);
    }
  }

  /**
   * @private
   * @param {Array} array
   * @param {number[]} criteria
   */
  function sortByCriteria(array, criteria) {
    var l = array.length;
    var indices = Array(l);
    var i;
    for (i = 0; i < l; i++) {
      indices[i] = i;
    }
    quickSort(criteria, 0, l - 1, indices);
    var result = Array(l);
    for (var n = 0; n < l; n++) {
      i = indices[n];
      result[n] = i === undefined ? array[n] : array[i];
    }
    return result;
  }

  function partition(array, i, j, mid, indices) {
    var l = i;
    var r = j;
    while (l <= r) {
      i = l;
      while (l < r && array[l] < mid) {
        l++;
      }
      while (r >= i && array[r] >= mid) {
        r--;
      }
      if (l > r) {
        break;
      }
      swap(array, indices, l++, r--);
    }
    return l;
  }

  function swap(array, indices, l, r) {
    var n = array[l];
    array[l] = array[r];
    array[r] = n;
    var i = indices[l];
    indices[l] = indices[r];
    indices[r] = i;
  }

  function quickSort(array, i, j, indices) {
    if (i === j) {
      return;
    }
    var k = i;
    while (++k <= j && array[i] === array[k]) {
      var l = k - 1;
      if (indices[l] > indices[k]) {
        var index = indices[l];
        indices[l] = indices[k];
        indices[k] = index;
      }
    }
    if (k > j) {
      return;
    }
    var p = array[i] > array[k] ? i : k;
    k = partition(array, i, j, array[p], indices);
    quickSort(array, i, k - 1, indices);
    quickSort(array, k, j, indices);
  }

  /**
   * @Private
   */
  function makeConcatResult(array) {
    var result = [];
    arrayEachSync(array, function(value) {
      if (value === noop) {
        return;
      }
      if (isArray(value)) {
        nativePush.apply(result, value);
      } else {
        result.push(value);
      }
    });
    return result;
  }

  /* async functions */

  /**
   * @private
   */
  function arrayEach(array, iterator, callback) {
    var index = -1;
    var size = array.length;

    if (iterator.length === 3) {
      while (++index < size) {
        iterator(array[index], index, onlyOnce(callback));
      }
    } else {
      while (++index < size) {
        iterator(array[index], onlyOnce(callback));
      }
    }
  }

  /**
   * @private
   */
  function baseEach(object, iterator, callback, keys) {
    var key;
    var index = -1;
    var size = keys.length;

    if (iterator.length === 3) {
      while (++index < size) {
        key = keys[index];
        iterator(object[key], key, onlyOnce(callback));
      }
    } else {
      while (++index < size) {
        iterator(object[keys[index]], onlyOnce(callback));
      }
    }
  }

  /**
   * @private
   */
  function symbolEach(collection, iterator, callback) {
    var iter = collection[iteratorSymbol]();
    var index = 0;
    var item;
    if (iterator.length === 3) {
      while ((item = iter.next()).done === false) {
        iterator(item.value, index++, onlyOnce(callback));
      }
    } else {
      while ((item = iter.next()).done === false) {
        index++;
        iterator(item.value, onlyOnce(callback));
      }
    }
    return index;
  }

  /**
   * @private
   */
  function arrayEachResult(array, result, iterator, callback) {
    var index = -1;
    var size = array.length;

    if (iterator.length === 4) {
      while (++index < size) {
        iterator(result, array[index], index, onlyOnce(callback));
      }
    } else {
      while (++index < size) {
        iterator(result, array[index], onlyOnce(callback));
      }
    }
  }

  /**
   * @private
   */
  function baseEachResult(object, result, iterator, callback, keys) {
    var key;
    var index = -1;
    var size = keys.length;

    if (iterator.length === 4) {
      while (++index < size) {
        key = keys[index];
        iterator(result, object[key], key, onlyOnce(callback));
      }
    } else {
      while (++index < size) {
        iterator(result, object[keys[index]], onlyOnce(callback));
      }
    }
  }

  /**
   * @private
   */
  function symbolEachResult(collection, result, iterator, callback) {
    var item;
    var index = 0;
    var iter = collection[iteratorSymbol]();

    if (iterator.length === 4) {
      while ((item = iter.next()).done === false) {
        iterator(result, item.value, index++, onlyOnce(callback));
      }
    } else {
      while ((item = iter.next()).done === false) {
        index++;
        iterator(result, item.value, onlyOnce(callback));
      }
    }
    return index;
  }

  /**
   * @private
   */
  function arrayEachFunc(array, createCallback) {
    var index = -1;
    var size = array.length;

    while (++index < size) {
      array[index](createCallback(index));
    }
  }

  /**
   * @private
   */
  function baseEachFunc(object, createCallback, keys) {
    var key;
    var index = -1;
    var size = keys.length;

    while (++index < size) {
      key = keys[index];
      object[key](createCallback(key));
    }
  }

  /**
   * @private
   */
  function arrayEachIndex(array, iterator, createCallback) {
    var index = -1;
    var size = array.length;

    if (iterator.length === 3) {
      while (++index < size) {
        iterator(array[index], index, createCallback(index));
      }
    } else {
      while (++index < size) {
        iterator(array[index], createCallback(index));
      }
    }
  }

  /**
   * @private
   */
  function baseEachIndex(object, iterator, createCallback, keys) {
    var key;
    var index = -1;
    var size = keys.length;

    if (iterator.length === 3) {
      while (++index < size) {
        key = keys[index];
        iterator(object[key], key, createCallback(index));
      }
    } else {
      while (++index < size) {
        iterator(object[keys[index]], createCallback(index));
      }
    }
  }

  /**
   * @private
   */
  function symbolEachIndex(collection, iterator, createCallback) {
    var item;
    var index = 0;
    var iter = collection[iteratorSymbol]();

    if (iterator.length === 3) {
      while ((item = iter.next()).done === false) {
        iterator(item.value, index, createCallback(index++));
      }
    } else {
      while ((item = iter.next()).done === false) {
        iterator(item.value, createCallback(index++));
      }
    }
    return index;
  }

  /**
   * @private
   */
  function baseEachKey(object, iterator, createCallback, keys) {
    var key;
    var index = -1;
    var size = keys.length;

    if (iterator.length === 3) {
      while (++index < size) {
        key = keys[index];
        iterator(object[key], key, createCallback(key));
      }
    } else {
      while (++index < size) {
        key = keys[index];
        iterator(object[key], createCallback(key));
      }
    }
  }

  /**
   * @private
   */
  function symbolEachKey(collection, iterator, createCallback) {
    var item;
    var index = 0;
    var iter = collection[iteratorSymbol]();

    if (iterator.length === 3) {
      while ((item = iter.next()).done === false) {
        iterator(item.value, index, createCallback(index++));
      }
    } else {
      while ((item = iter.next()).done === false) {
        iterator(item.value, createCallback(index++));
      }
    }
    return index;
  }

  /**
   * @private
   */
  function arrayEachValue(array, iterator, createCallback) {
    var value;
    var index = -1;
    var size = array.length;

    if (iterator.length === 3) {
      while (++index < size) {
        value = array[index];
        iterator(value, index, createCallback(value));
      }
    } else {
      while (++index < size) {
        value = array[index];
        iterator(value, createCallback(value));
      }
    }
  }

  /**
   * @private
   */
  function baseEachValue(object, iterator, createCallback, keys) {
    var key, value;
    var index = -1;
    var size = keys.length;

    if (iterator.length === 3) {
      while (++index < size) {
        key = keys[index];
        value = object[key];
        iterator(value, key, createCallback(value));
      }
    } else {
      while (++index < size) {
        value = object[keys[index]];
        iterator(value, createCallback(value));
      }
    }
  }

  /**
   * @private
   */
  function symbolEachValue(collection, iterator, createCallback) {
    var value, item;
    var index = 0;
    var iter = collection[iteratorSymbol]();

    if (iterator.length === 3) {
      while ((item = iter.next()).done === false) {
        value = item.value;
        iterator(value, index++, createCallback(value));
      }
    } else {
      while ((item = iter.next()).done === false) {
        index++;
        value = item.value;
        iterator(value, createCallback(value));
      }
    }
    return index;
  }

  /**
   * @private
   */
  function arrayEachIndexValue(array, iterator, createCallback) {
    var value;
    var index = -1;
    var size = array.length;

    if (iterator.length === 3) {
      while (++index < size) {
        value = array[index];
        iterator(value, index, createCallback(index, value));
      }
    } else {
      while (++index < size) {
        value = array[index];
        iterator(value, createCallback(index, value));
      }
    }
  }

  /**
   * @private
   */
  function baseEachIndexValue(object, iterator, createCallback, keys) {
    var key, value;
    var index = -1;
    var size = keys.length;

    if (iterator.length === 3) {
      while (++index < size) {
        key = keys[index];
        value = object[key];
        iterator(value, key, createCallback(index, value));
      }
    } else {
      while (++index < size) {
        value = object[keys[index]];
        iterator(value, createCallback(index, value));
      }
    }
  }

  /**
   * @private
   */
  function symbolEachIndexValue(collection, iterator, createCallback) {
    var value, item;
    var index = 0;
    var iter = collection[iteratorSymbol]();

    if (iterator.length === 3) {
      while ((item = iter.next()).done === false) {
        value = item.value;
        iterator(value, index, createCallback(index++, value));
      }
    } else {
      while ((item = iter.next()).done === false) {
        value = item.value;
        iterator(value, createCallback(index++, value));
      }
    }
    return index;
  }

  /**
   * @private
   */
  function baseEachKeyValue(object, iterator, createCallback, keys) {
    var key, value;
    var index = -1;
    var size = keys.length;

    if (iterator.length === 3) {
      while (++index < size) {
        key = keys[index];
        value = object[key];
        iterator(value, key, createCallback(key, value));
      }
    } else {
      while (++index < size) {
        key = keys[index];
        value = object[key];
        iterator(value, createCallback(key, value));
      }
    }
  }

  /**
   * @private
   */
  function symbolEachKeyValue(collection, iterator, createCallback) {
    var value, item;
    var index = 0;
    var iter = collection[iteratorSymbol]();

    if (iterator.length === 3) {
      while ((item = iter.next()).done === false) {
        value = item.value;
        iterator(value, index, createCallback(index++, value));
      }
    } else {
      while ((item = iter.next()).done === false) {
        value = item.value;
        iterator(value, createCallback(index++, value));
      }
    }
    return index;
  }

  /**
   * @private
   * @param {Function} func
   */
  function onlyOnce(func) {
    return function(err, res) {
      var fn = func;
      func = throwError;
      fn(err, res);
    };
  }

  /**
   * @private
   * @param {Function} func
   */
  function once(func) {
    return function(err, res) {
      var fn = func;
      func = noop;
      fn(err, res);
    };
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   */
  function createEach(arrayEach, baseEach, symbolEach) {
    return function each(collection, iterator, callback) {
      callback = once(callback || noop);
      var size, keys;
      var completed = 0;
      if (isArray(collection)) {
        size = collection.length;
        arrayEach(collection, iterator, done);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = symbolEach(collection, iterator, done);
        size && size === completed && callback(null);
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        baseEach(collection, iterator, done, keys);
      }
      if (!size) {
        callback(null);
      }

      function done(err, bool) {
        if (err) {
          callback = once(callback);
          callback(err);
        } else if (++completed === size) {
          callback(null);
        } else if (bool === false) {
          callback = once(callback);
          callback(null);
        }
      }
    };
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   */
  function createMap(arrayEach, baseEach, symbolEach, useArray) {
    var init, clone;
    if (useArray) {
      init = Array;
      clone = createArray;
    } else {
      init = function() {
        return {};
      };
      clone = objectClone;
    }

    return function(collection, iterator, callback) {
      callback = callback || noop;
      var size, keys, result;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        result = init(size);
        arrayEach(collection, iterator, createCallback);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        // TODO: size could be changed
        result = init(0);
        size = symbolEach(collection, iterator, createCallback);
        size && size === completed && callback(null, result);
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        result = init(size);
        baseEach(collection, iterator, createCallback, keys);
      }
      if (!size) {
        callback(null, init());
      }

      function createCallback(key) {
        return function done(err, res) {
          if (key === null) {
            throwError();
          }
          if (err) {
            key = null;
            callback = once(callback);
            callback(err, clone(result));
            return;
          }
          result[key] = res;
          key = null;
          if (++completed === size) {
            callback(null, result);
          }
        };
      }
    };
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   * @param {boolean} bool
   */
  function createFilter(arrayEach, baseEach, symbolEach, bool) {
    return function(collection, iterator, callback) {
      callback = callback || noop;
      var size, keys, result;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        result = Array(size);
        arrayEach(collection, iterator, createCallback);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        result = [];
        size = symbolEach(collection, iterator, createCallback);
        size && size === completed && callback(null, compact(result));
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        result = Array(size);
        baseEach(collection, iterator, createCallback, keys);
      }
      if (!size) {
        return callback(null, []);
      }

      function createCallback(index, value) {
        return function done(err, res) {
          if (index === null) {
            throwError();
          }
          if (err) {
            index = null;
            callback = once(callback);
            callback(err);
            return;
          }
          if (!!res === bool) {
            result[index] = value;
          }
          index = null;
          if (++completed === size) {
            callback(null, compact(result));
          }
        };
      }
    };
  }

  /**
   * @private
   * @param {boolean} bool
   */
  function createFilterSeries(bool) {
    return function(collection, iterator, callback) {
      callback = onlyOnce(callback || noop);
      var size, key, value, keys, iter, item, iterate;
      var sync = false;
      var completed = 0;
      var result = [];

      if (isArray(collection)) {
        size = collection.length;
        iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = Infinity;
        iter = collection[iteratorSymbol]();
        iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
      }
      if (!size) {
        return callback(null, []);
      }
      iterate();

      function arrayIterator() {
        value = collection[completed];
        iterator(value, done);
      }

      function arrayIteratorWithIndex() {
        value = collection[completed];
        iterator(value, completed, done);
      }

      function symbolIterator() {
        item = iter.next();
        value = item.value;
        item.done ? callback(null, result) : iterator(value, done);
      }

      function symbolIteratorWithKey() {
        item = iter.next();
        value = item.value;
        item.done ? callback(null, result) : iterator(value, completed, done);
      }

      function objectIterator() {
        key = keys[completed];
        value = collection[key];
        iterator(value, done);
      }

      function objectIteratorWithKey() {
        key = keys[completed];
        value = collection[key];
        iterator(value, key, done);
      }

      function done(err, res) {
        if (err) {
          callback(err);
          return;
        }
        if (!!res === bool) {
          result[result.length] = value;
        }
        if (++completed === size) {
          iterate = throwError;
          callback(null, result);
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      }
    };
  }

  /**
   * @private
   * @param {boolean} bool
   */
  function createFilterLimit(bool) {
    return function(collection, limit, iterator, callback) {
      callback = callback || noop;
      var size, index, key, value, keys, iter, item, iterate, result;
      var sync = false;
      var started = 0;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = Infinity;
        result = [];
        iter = collection[iteratorSymbol]();
        iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
      }
      if (!size || isNaN(limit) || limit < 1) {
        return callback(null, []);
      }
      result = result || Array(size);
      timesSync(limit > size ? size : limit, iterate);

      function arrayIterator() {
        index = started++;
        if (index < size) {
          value = collection[index];
          iterator(value, createCallback(value, index));
        }
      }

      function arrayIteratorWithIndex() {
        index = started++;
        if (index < size) {
          value = collection[index];
          iterator(value, index, createCallback(value, index));
        }
      }

      function symbolIterator() {
        item = iter.next();
        if (item.done === false) {
          value = item.value;
          iterator(value, createCallback(value, started++));
        } else if (completed === started && iterator !== noop) {
          iterator = noop;
          callback(null, compact(result));
        }
      }

      function symbolIteratorWithKey() {
        item = iter.next();
        if (item.done === false) {
          value = item.value;
          iterator(value, started, createCallback(value, started++));
        } else if (completed === started && iterator !== noop) {
          iterator = noop;
          callback(null, compact(result));
        }
      }

      function objectIterator() {
        index = started++;
        if (index < size) {
          value = collection[keys[index]];
          iterator(value, createCallback(value, index));
        }
      }

      function objectIteratorWithKey() {
        index = started++;
        if (index < size) {
          key = keys[index];
          value = collection[key];
          iterator(value, key, createCallback(value, index));
        }
      }

      function createCallback(value, index) {
        return function(err, res) {
          if (index === null) {
            throwError();
          }
          if (err) {
            index = null;
            iterate = noop;
            callback = once(callback);
            callback(err);
            return;
          }
          if (!!res === bool) {
            result[index] = value;
          }
          index = null;
          if (++completed === size) {
            callback = onlyOnce(callback);
            callback(null, compact(result));
          } else if (sync) {
            nextTick(iterate);
          } else {
            sync = true;
            iterate();
          }
          sync = false;
        };
      }
    };
  }

  /**
   * @memberof async
   * @namespace eachSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.eachSeries(array, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done();
   *   }, num * 10);
   * };
   * async.eachSeries(array, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [[1, 0], [3, 1], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.eachSeries(object, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done();
   *   }, num * 10);
   * };
   * async.eachSeries(object, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'b']]
   * });
   *
   * @example
   *
   * // break
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num !== 3);
   *   }, num * 10);
   * };
   * async.eachSeries(array, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 3]
   * });
   */
  function eachSeries(collection, iterator, callback) {
    callback = onlyOnce(callback || noop);
    var size, key, keys, iter, item, iterate;
    var sync = false;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null);
    }
    iterate();

    function arrayIterator() {
      iterator(collection[completed], done);
    }

    function arrayIteratorWithIndex() {
      iterator(collection[completed], completed, done);
    }

    function symbolIterator() {
      item = iter.next();
      item.done ? callback(null) : iterator(item.value, done);
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      item.done ? callback(null) : iterator(item.value, completed, done);
    }

    function objectIterator() {
      iterator(collection[keys[completed]], done);
    }

    function objectIteratorWithKey() {
      key = keys[completed];
      iterator(collection[key], key, done);
    }

    function done(err, bool) {
      if (err) {
        callback(err);
      } else if (++completed === size || bool === false) {
        iterate = throwError;
        callback(null);
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace eachLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.eachLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done();
   *   }, num * 10);
   * };
   * async.eachLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.eachLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done();
   *   }, num * 10);
   * };
   * async.eachLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   * @example
   *
   * // break
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num !== 5);
   *   }, num * 10);
   * };
   * async.eachLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // undefined
   *   console.log(order); // [1, 3, 5]
   * });
   *
   */
  function eachLimit(collection, limit, iterator, callback) {
    callback = callback || noop;
    var size, index, key, keys, iter, item, iterate;
    var sync = false;
    var started = 0;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    } else {
      return callback(null);
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null);
    }
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      if (started < size) {
        iterator(collection[started++], done);
      }
    }

    function arrayIteratorWithIndex() {
      index = started++;
      if (index < size) {
        iterator(collection[index], index, done);
      }
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done === false) {
        started++;
        iterator(item.value, done);
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null);
      }
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done === false) {
        iterator(item.value, started++, done);
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null);
      }
    }

    function objectIterator() {
      if (started < size) {
        iterator(collection[keys[started++]], done);
      }
    }

    function objectIteratorWithKey() {
      index = started++;
      if (index < size) {
        key = keys[index];
        iterator(collection[key], key, done);
      }
    }

    function done(err, bool) {
      if (err || bool === false) {
        iterate = noop;
        callback = once(callback);
        callback(err);
      } else if (++completed === size) {
        iterator = noop;
        iterate = throwError;
        callback = onlyOnce(callback);
        callback(null);
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace mapSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapSeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2];
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapSeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [[1, 0], [3, 1], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapSeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapSeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c']]
   * });
   *
   */
  function mapSeries(collection, iterator, callback) {
    callback = callback || noop;
    var size, key, keys, iter, item, result, iterate;
    var sync = false;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      result = [];
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null, []);
    }
    result = result || Array(size);
    iterate();

    function arrayIterator() {
      iterator(collection[completed], done);
    }

    function arrayIteratorWithIndex() {
      iterator(collection[completed], completed, done);
    }

    function symbolIterator() {
      item = iter.next();
      item.done ? callback(null, result) : iterator(item.value, done);
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      item.done ? callback(null, result) : iterator(item.value, completed, done);
    }

    function objectIterator() {
      iterator(collection[keys[completed]], done);
    }

    function objectIteratorWithKey() {
      key = keys[completed];
      iterator(collection[key], key, done);
    }

    function done(err, res) {
      if (err) {
        iterate = throwError;
        callback = onlyOnce(callback);
        callback(err, createArray(result));
        return;
      }
      result[completed] = res;
      if (++completed === size) {
        iterate = throwError;
        callback(null, result);
        callback = throwError;
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace mapLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3, 4, 2]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3, 4, 2]
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3, 4, 2]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 5, 3, 4, 2]
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  function mapLimit(collection, limit, iterator, callback) {
    callback = callback || noop;
    var size, index, key, keys, iter, item, result, iterate;
    var sync = false;
    var started = 0;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      result = [];
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null, []);
    }
    result = result || Array(size);
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      index = started++;
      if (index < size) {
        iterator(collection[index], createCallback(index));
      }
    }

    function arrayIteratorWithIndex() {
      index = started++;
      if (index < size) {
        iterator(collection[index], index, createCallback(index));
      }
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done === false) {
        iterator(item.value, createCallback(started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done === false) {
        iterator(item.value, started, createCallback(started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function objectIterator() {
      index = started++;
      if (index < size) {
        iterator(collection[keys[index]], createCallback(index));
      }
    }

    function objectIteratorWithKey() {
      index = started++;
      if (index < size) {
        key = keys[index];
        iterator(collection[key], key, createCallback(index));
      }
    }

    function createCallback(index) {
      return function(err, res) {
        if (index === null) {
          throwError();
        }
        if (err) {
          index = null;
          iterate = noop;
          callback = once(callback);
          callback(err, createArray(result));
          return;
        }
        result[index] = res;
        index = null;
        if (++completed === size) {
          iterate = throwError;
          callback(null, result);
          callback = throwError;
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      };
    }
  }

  /**
   * @memberof async
   * @namespace mapValuesSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesSeries(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3, '2': 2 }
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesSeries(array, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3, '2': 2 }
   *   console.log(order); // [[1, 0], [3, 1], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesSeries(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3, c: 2 }
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesSeries(object, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 3, c: 2 }
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c']]
   * });
   *
   */
  function mapValuesSeries(collection, iterator, callback) {
    callback = callback || noop;
    var size, key, keys, iter, item, iterate;
    var sync = false;
    var result = {};
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null, result);
    }
    iterate();

    function arrayIterator() {
      key = completed;
      iterator(collection[completed], done);
    }

    function arrayIteratorWithIndex() {
      key = completed;
      iterator(collection[completed], completed, done);
    }

    function symbolIterator() {
      key = completed;
      item = iter.next();
      item.done ? callback(null, result) : iterator(item.value, done);
    }

    function symbolIteratorWithKey() {
      key = completed;
      item = iter.next();
      item.done ? callback(null, result) : iterator(item.value, completed, done);
    }

    function objectIterator() {
      key = keys[completed];
      iterator(collection[key], done);
    }

    function objectIteratorWithKey() {
      key = keys[completed];
      iterator(collection[key], key, done);
    }

    function done(err, res) {
      if (err) {
        iterate = throwError;
        callback = onlyOnce(callback);
        callback(err, objectClone(result));
        return;
      }
      result[key] = res;
      if (++completed === size) {
        iterate = throwError;
        callback(null, result);
        callback = throwError;
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace mapValuesLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 5, '2': 3, '3': 4, '4': 2 }
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 5, '2': 3, '3': 4, '4': 2 }
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 5, c: 3, d: 4, e: 2 }
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.mapValuesLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 5, c: 3, d: 4, e: 2 }
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  function mapValuesLimit(collection, limit, iterator, callback) {
    callback = callback || noop;
    var size, index, key, keys, iter, item, iterate;
    var sync = false;
    var result = {};
    var started = 0;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null, result);
    }
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      index = started++;
      if (index < size) {
        iterator(collection[index], createCallback(index));
      }
    }

    function arrayIteratorWithIndex() {
      index = started++;
      if (index < size) {
        iterator(collection[index], index, createCallback(index));
      }
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done === false) {
        iterator(item.value, createCallback(started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done === false) {
        iterator(item.value, started, createCallback(started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function objectIterator() {
      index = started++;
      if (index < size) {
        key = keys[index];
        iterator(collection[key], createCallback(key));
      }
    }

    function objectIteratorWithKey() {
      index = started++;
      if (index < size) {
        key = keys[index];
        iterator(collection[key], key, createCallback(key));
      }
    }

    function createCallback(key) {
      return function(err, res) {
        if (key === null) {
          throwError();
        }
        if (err) {
          key = null;
          iterate = noop;
          callback = once(callback);
          callback(err, objectClone(result));
          return;
        }
        result[key] = res;
        key = null;
        if (++completed === size) {
          callback(null, result);
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      };
    }
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   * @param {boolean} bool
   */
  function createDetect(arrayEach, baseEach, symbolEach, bool) {
    return function(collection, iterator, callback) {
      callback = callback || noop;
      var size, keys;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        arrayEach(collection, iterator, createCallback);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = symbolEach(collection, iterator, createCallback);
        size && size === completed && callback(null);
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        baseEach(collection, iterator, createCallback, keys);
      }
      if (!size) {
        callback(null);
      }

      function createCallback(value) {
        var called = false;
        return function done(err, res) {
          if (called) {
            throwError();
          }
          called = true;
          if (err) {
            callback = once(callback);
            callback(err);
          } else if (!!res === bool) {
            callback = once(callback);
            callback(null, value);
          } else if (++completed === size) {
            callback(null);
          }
        };
      }
    };
  }

  /**
   * @private
   * @param {boolean} bool
   */
  function createDetectSeries(bool) {
    return function(collection, iterator, callback) {
      callback = onlyOnce(callback || noop);
      var size, key, value, keys, iter, item, iterate;
      var sync = false;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = Infinity;
        iter = collection[iteratorSymbol]();
        iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
      }
      if (!size) {
        return callback(null);
      }
      iterate();

      function arrayIterator() {
        value = collection[completed];
        iterator(value, done);
      }

      function arrayIteratorWithIndex() {
        value = collection[completed];
        iterator(value, completed, done);
      }

      function symbolIterator() {
        item = iter.next();
        value = item.value;
        item.done ? callback(null) : iterator(value, done);
      }

      function symbolIteratorWithKey() {
        item = iter.next();
        value = item.value;
        item.done ? callback(null) : iterator(value, completed, done);
      }

      function objectIterator() {
        value = collection[keys[completed]];
        iterator(value, done);
      }

      function objectIteratorWithKey() {
        key = keys[completed];
        value = collection[key];
        iterator(value, key, done);
      }

      function done(err, res) {
        if (err) {
          callback(err);
        } else if (!!res === bool) {
          iterate = throwError;
          callback(null, value);
        } else if (++completed === size) {
          iterate = throwError;
          callback(null);
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      }
    };
  }

  /**
   * @private
   * @param {boolean} bool
   */
  function createDetectLimit(bool) {
    return function(collection, limit, iterator, callback) {
      callback = callback || noop;
      var size, index, key, value, keys, iter, item, iterate;
      var sync = false;
      var started = 0;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = Infinity;
        iter = collection[iteratorSymbol]();
        iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
      }
      if (!size || isNaN(limit) || limit < 1) {
        return callback(null);
      }
      timesSync(limit > size ? size : limit, iterate);

      function arrayIterator() {
        index = started++;
        if (index < size) {
          value = collection[index];
          iterator(value, createCallback(value));
        }
      }

      function arrayIteratorWithIndex() {
        index = started++;
        if (index < size) {
          value = collection[index];
          iterator(value, index, createCallback(value));
        }
      }

      function symbolIterator() {
        item = iter.next();
        if (item.done === false) {
          started++;
          value = item.value;
          iterator(value, createCallback(value));
        } else if (completed === started && iterator !== noop) {
          iterator = noop;
          callback(null);
        }
      }

      function symbolIteratorWithKey() {
        item = iter.next();
        if (item.done === false) {
          value = item.value;
          iterator(value, started++, createCallback(value));
        } else if (completed === started && iterator !== noop) {
          iterator = noop;
          callback(null);
        }
      }

      function objectIterator() {
        index = started++;
        if (index < size) {
          value = collection[keys[index]];
          iterator(value, createCallback(value));
        }
      }

      function objectIteratorWithKey() {
        if (started < size) {
          key = keys[started++];
          value = collection[key];
          iterator(value, key, createCallback(value));
        }
      }

      function createCallback(value) {
        var called = false;
        return function(err, res) {
          if (called) {
            throwError();
          }
          called = true;
          if (err) {
            iterate = noop;
            callback = once(callback);
            callback(err);
          } else if (!!res === bool) {
            iterate = noop;
            callback = once(callback);
            callback(null, value);
          } else if (++completed === size) {
            callback(null);
          } else if (sync) {
            nextTick(iterate);
          } else {
            sync = true;
            iterate();
          }
          sync = false;
        };
      }
    };
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   * @param {boolean} bool
   */
  function createPick(arrayEach, baseEach, symbolEach, bool) {
    return function(collection, iterator, callback) {
      callback = callback || noop;
      var size, keys;
      var completed = 0;
      var result = {};

      if (isArray(collection)) {
        size = collection.length;
        arrayEach(collection, iterator, createCallback);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = symbolEach(collection, iterator, createCallback);
        size && size === completed && callback(null, result);
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        baseEach(collection, iterator, createCallback, keys);
      }
      if (!size) {
        return callback(null, {});
      }

      function createCallback(key, value) {
        return function done(err, res) {
          if (key === null) {
            throwError();
          }
          if (err) {
            key = null;
            callback = once(callback);
            callback(err, objectClone(result));
            return;
          }
          if (!!res === bool) {
            result[key] = value;
          }
          key = null;
          if (++completed === size) {
            callback(null, result);
          }
        };
      }
    };
  }

  /**
   * @private
   * @param {boolean} bool
   */
  function createPickSeries(bool) {
    return function(collection, iterator, callback) {
      callback = onlyOnce(callback || noop);
      var size, key, value, keys, iter, item, iterate;
      var sync = false;
      var result = {};
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = Infinity;
        iter = collection[iteratorSymbol]();
        iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
      }
      if (!size) {
        return callback(null, {});
      }
      iterate();

      function arrayIterator() {
        key = completed;
        value = collection[completed];
        iterator(value, done);
      }

      function arrayIteratorWithIndex() {
        key = completed;
        value = collection[completed];
        iterator(value, completed, done);
      }

      function symbolIterator() {
        key = completed;
        item = iter.next();
        value = item.value;
        item.done ? callback(null, result) : iterator(value, done);
      }

      function symbolIteratorWithKey() {
        key = completed;
        item = iter.next();
        value = item.value;
        item.done ? callback(null, result) : iterator(value, key, done);
      }

      function objectIterator() {
        key = keys[completed];
        value = collection[key];
        iterator(value, done);
      }

      function objectIteratorWithKey() {
        key = keys[completed];
        value = collection[key];
        iterator(value, key, done);
      }

      function done(err, res) {
        if (err) {
          callback(err, result);
          return;
        }
        if (!!res === bool) {
          result[key] = value;
        }
        if (++completed === size) {
          iterate = throwError;
          callback(null, result);
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      }
    };
  }

  /**
   * @private
   * @param {boolean} bool
   */
  function createPickLimit(bool) {
    return function(collection, limit, iterator, callback) {
      callback = callback || noop;
      var size, index, key, value, keys, iter, item, iterate;
      var sync = false;
      var result = {};
      var started = 0;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = Infinity;
        iter = collection[iteratorSymbol]();
        iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
      }
      if (!size || isNaN(limit) || limit < 1) {
        return callback(null, {});
      }
      timesSync(limit > size ? size : limit, iterate);

      function arrayIterator() {
        index = started++;
        if (index < size) {
          value = collection[index];
          iterator(value, createCallback(value, index));
        }
      }

      function arrayIteratorWithIndex() {
        index = started++;
        if (index < size) {
          value = collection[index];
          iterator(value, index, createCallback(value, index));
        }
      }

      function symbolIterator() {
        item = iter.next();
        if (item.done === false) {
          value = item.value;
          iterator(value, createCallback(value, started++));
        } else if (completed === started && iterator !== noop) {
          iterator = noop;
          callback(null, result);
        }
      }

      function symbolIteratorWithKey() {
        item = iter.next();
        if (item.done === false) {
          value = item.value;
          iterator(value, started, createCallback(value, started++));
        } else if (completed === started && iterator !== noop) {
          iterator = noop;
          callback(null, result);
        }
      }

      function objectIterator() {
        if (started < size) {
          key = keys[started++];
          value = collection[key];
          iterator(value, createCallback(value, key));
        }
      }

      function objectIteratorWithKey() {
        if (started < size) {
          key = keys[started++];
          value = collection[key];
          iterator(value, key, createCallback(value, key));
        }
      }

      function createCallback(value, key) {
        return function(err, res) {
          if (key === null) {
            throwError();
          }
          if (err) {
            key = null;
            iterate = noop;
            callback = once(callback);
            callback(err, objectClone(result));
            return;
          }
          if (!!res === bool) {
            result[key] = value;
          }
          key = null;
          if (++completed === size) {
            iterate = throwError;
            callback = onlyOnce(callback);
            callback(null, result);
          } else if (sync) {
            nextTick(iterate);
          } else {
            sync = true;
            iterate();
          }
          sync = false;
        };
      }
    };
  }

  /**
   * @memberof async
   * @namespace reduce
   * @param {Array|Object} collection
   * @param {*} result
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduce(collection, 0, iterator, function(err, res) {
   *   console.log(res); // 10
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduce(collection, '', iterator, function(err, res) {
   *   console.log(res); // '1324'
   *   console.log(order); // [[1, 0], [3, 1], [2, 2], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduce(collection, '', iterator, function(err, res) {
   *   console.log(res); // '1324'
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduce(collection, 0, iterator, function(err, res) {
   *   console.log(res); // 10
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'b'], [4, 'd']]
   * });
   *
   */
  function reduce(collection, result, iterator, callback) {
    callback = onlyOnce(callback || noop);
    var size, key, keys, iter, item, iterate;
    var sync = false;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 4 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 4 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 4 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null, result);
    }
    iterate(result);

    function arrayIterator(result) {
      iterator(result, collection[completed], done);
    }

    function arrayIteratorWithIndex(result) {
      iterator(result, collection[completed], completed, done);
    }

    function symbolIterator(result) {
      item = iter.next();
      item.done ? callback(null, result) : iterator(result, item.value, done);
    }

    function symbolIteratorWithKey(result) {
      item = iter.next();
      item.done ? callback(null, result) : iterator(result, item.value, completed, done);
    }

    function objectIterator(result) {
      iterator(result, collection[keys[completed]], done);
    }

    function objectIteratorWithKey(result) {
      key = keys[completed];
      iterator(result, collection[key], key, done);
    }

    function done(err, result) {
      if (err) {
        callback(err, result);
      } else if (++completed === size) {
        iterator = throwError;
        callback(null, result);
      } else if (sync) {
        nextTick(function() {
          iterate(result);
        });
      } else {
        sync = true;
        iterate(result);
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace reduceRight
   * @param {Array|Object} collection
   * @param {*} result
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduceRight(collection, 0, iterator, function(err, res) {
   *   console.log(res); // 10
   *   console.log(order); // [4, 2, 3, 1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduceRight(collection, '', iterator, function(err, res) {
   *   console.log(res); // '4231'
   *   console.log(order); // [[4, 3], [2, 2], [3, 1], [1, 0]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduceRight(collection, '', iterator, function(err, res) {
   *   console.log(res); // '4231'
   *   console.log(order); // [4, 2, 3, 1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, result + num);
   *   }, num * 10);
   * };
   * async.reduceRight(collection, 0, iterator, function(err, res) {
   *   console.log(res); // 10
   *   console.log(order); // [[4, 3], [2, 2], [3, 1], [1, 0]]
   * });
   *
   */
  function reduceRight(collection, result, iterator, callback) {
    callback = onlyOnce(callback || noop);
    var resIndex, index, key, keys, iter, item, col, iterate;
    var sync = false;

    if (isArray(collection)) {
      resIndex = collection.length;
      iterate = iterator.length === 4 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      col = [];
      iter = collection[iteratorSymbol]();
      index = -1;
      while ((item = iter.next()).done === false) {
        col[++index] = item.value;
      }
      collection = col;
      resIndex = col.length;
      iterate = iterator.length === 4 ? arrayIteratorWithIndex : arrayIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      resIndex = keys.length;
      iterate = iterator.length === 4 ? objectIteratorWithKey : objectIterator;
    }
    if (!resIndex) {
      return callback(null, result);
    }
    iterate(result);

    function arrayIterator(result) {
      iterator(result, collection[--resIndex], done);
    }

    function arrayIteratorWithIndex(result) {
      iterator(result, collection[--resIndex], resIndex, done);
    }

    function objectIterator(result) {
      iterator(result, collection[keys[--resIndex]], done);
    }

    function objectIteratorWithKey(result) {
      key = keys[--resIndex];
      iterator(result, collection[key], key, done);
    }

    function done(err, result) {
      if (err) {
        callback(err, result);
      } else if (resIndex === 0) {
        iterate = throwError;
        callback(null, result);
      } else if (sync) {
        nextTick(function() {
          iterate(result);
        });
      } else {
        sync = true;
        iterate(result);
      }
      sync = false;
    }
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   */
  function createTransform(arrayEach, baseEach, symbolEach) {
    return function transform(collection, accumulator, iterator, callback) {
      if (arguments.length === 3) {
        callback = iterator;
        iterator = accumulator;
        accumulator = undefined;
      }
      callback = callback || noop;
      var size, keys, result;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        result = accumulator !== undefined ? accumulator : [];
        arrayEach(collection, result, iterator, done);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        result = accumulator !== undefined ? accumulator : {};
        size = symbolEach(collection, result, iterator, done);
        size && size === completed && callback(null, result);
      } else if (typeof collection === obj) {
        keys = nativeKeys(collection);
        size = keys.length;
        result = accumulator !== undefined ? accumulator : {};
        baseEach(collection, result, iterator, done, keys);
      }
      if (!size) {
        callback(null, accumulator !== undefined ? accumulator : result || {});
      }

      function done(err, bool) {
        if (err) {
          callback = once(callback);
          callback(err, isArray(result) ? createArray(result) : objectClone(result));
        } else if (++completed === size) {
          callback(null, result);
        } else if (bool === false) {
          callback = once(callback);
          callback(null, isArray(result) ? createArray(result) : objectClone(result));
        }
      }
    };
  }

  /**
   * @memberof async
   * @namespace transformSeries
   * @param {Array|Object} collection
   * @param {Array|Object|Function} [accumulator]
   * @param {Function} [iterator]
   * @param {Function} [callback]
   * @example
   *
   * // array
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     result.push(num)
   *     done();
   *   }, num * 10);
   * };
   * async.transformSeries(collection, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2, 4]
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // array with index and accumulator
   * var order = [];
   * var collection = [1, 3, 2, 4];
   * var iterator = function(result, num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     result[index] = num;
   *     done();
   *   }, num * 10);
   * };
   * async.transformSeries(collection, {}, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 3, '2': 2, '3': 4 }
   *   console.log(order); // [[1, 0], [3, 1], [2, 2], [4, 3]]
   * });
   *
   * @example
   *
   * // object with accumulator
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     result.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.transformSeries(collection, [], iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2, 4]
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2, d: 4 };
   * var iterator = function(result, num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     result[key] = num;
   *     done();
   *   }, num * 10);
   * };
   * async.transformSeries(collection, iterator, function(err, res) {
   *   console.log(res); //  { a: 1, b: 3, c: 2, d: 4 }
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'b'], [4, 'd']]
   * });
   *
   */
  function transformSeries(collection, accumulator, iterator, callback) {
    if (arguments.length === 3) {
      callback = iterator;
      iterator = accumulator;
      accumulator = undefined;
    }
    callback = onlyOnce(callback || noop);
    var size, key, keys, iter, item, iterate, result;
    var sync = false;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      result = accumulator !== undefined ? accumulator : [];
      iterate = iterator.length === 4 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      result = accumulator !== undefined ? accumulator : {};
      iterate = iterator.length === 4 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      result = accumulator !== undefined ? accumulator : {};
      iterate = iterator.length === 4 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null, accumulator !== undefined ? accumulator : result || {});
    }
    iterate();

    function arrayIterator() {
      iterator(result, collection[completed], done);
    }

    function arrayIteratorWithIndex() {
      iterator(result, collection[completed], completed, done);
    }

    function symbolIterator() {
      item = iter.next();
      item.done ? callback(null, result) : iterator(result, item.value, done);
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      item.done ? callback(null, result) : iterator(result, item.value, completed, done);
    }

    function objectIterator() {
      iterator(result, collection[keys[completed]], done);
    }

    function objectIteratorWithKey() {
      key = keys[completed];
      iterator(result, collection[key], key, done);
    }

    function done(err, bool) {
      if (err) {
        callback(err, result);
      } else if (++completed === size || bool === false) {
        iterate = throwError;
        callback(null, result);
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace transformLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Array|Object|Function} [accumulator]
   * @param {Function} [iterator]
   * @param {Function} [callback]
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     result.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.transformLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 5, 2, 4]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index and accumulator
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(result, num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     result[index] = key;
   *     done();
   *   }, num * 10);
   * };
   * async.transformLimit(array, 2, {}, iterator, function(err, res) {
   *   console.log(res); // { '0': 1, '1': 5, '2': 3, '3': 4, '4': 2 }
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object with accumulator
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(result, num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     result.push(num);
   *     done();
   *   }, num * 10);
   * };
   * async.transformLimit(object, 2, [], iterator, function(err, res) {
   *   console.log(res); // [1, 3, 5, 2, 4]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(result, num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     result[key] = num;
   *     done();
   *   }, num * 10);
   * };
   * async.transformLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { a: 1, b: 5, c: 3, d: 4, e: 2 };
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  function transformLimit(collection, limit, accumulator, iterator, callback) {
    if (arguments.length === 4) {
      callback = iterator;
      iterator = accumulator;
      accumulator = undefined;
    }
    callback = callback || noop;
    var size, index, key, keys, iter, item, iterate, result;
    var sync = false;
    var started = 0;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      result = accumulator !== undefined ? accumulator : [];
      iterate = iterator.length === 4 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      result = accumulator !== undefined ? accumulator : {};
      iterate = iterator.length === 4 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      result = accumulator !== undefined ? accumulator : {};
      iterate = iterator.length === 4 ? objectIteratorWithKey : objectIterator;
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null, accumulator !== undefined ? accumulator : result || {});
    }
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      index = started++;
      if (index < size) {
        iterator(result, collection[index], onlyOnce(done));
      }
    }

    function arrayIteratorWithIndex() {
      index = started++;
      if (index < size) {
        iterator(result, collection[index], index, onlyOnce(done));
      }
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done === false) {
        started++;
        iterator(result, item.value, onlyOnce(done));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done === false) {
        iterator(result, item.value, started++, onlyOnce(done));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function objectIterator() {
      index = started++;
      if (index < size) {
        iterator(result, collection[keys[index]], onlyOnce(done));
      }
    }

    function objectIteratorWithKey() {
      index = started++;
      if (index < size) {
        key = keys[index];
        iterator(result, collection[key], key, onlyOnce(done));
      }
    }

    function done(err, bool) {
      if (err || bool === false) {
        iterate = noop;
        callback(err || null, isArray(result) ? createArray(result) : objectClone(result));
        callback = noop;
      } else if (++completed === size) {
        iterator = noop;
        callback(null, result);
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @private
   * @param {function} arrayEach
   * @param {function} baseEach
   * @param {function} symbolEach
   */
  function createSortBy(arrayEach, baseEach, symbolEach) {
    return function sortBy(collection, iterator, callback) {
      callback = callback || noop;
      var size, array, criteria;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        array = Array(size);
        criteria = Array(size);
        arrayEach(collection, iterator, createCallback);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        array = [];
        criteria = [];
        size = symbolEach(collection, iterator, createCallback);
        size && size === completed && callback(null, sortByCriteria(array, criteria));
      } else if (typeof collection === obj) {
        var keys = nativeKeys(collection);
        size = keys.length;
        array = Array(size);
        criteria = Array(size);
        baseEach(collection, iterator, createCallback, keys);
      }
      if (!size) {
        callback(null, []);
      }

      function createCallback(index, value) {
        var called = false;
        array[index] = value;
        return function done(err, criterion) {
          if (called) {
            throwError();
          }
          called = true;
          criteria[index] = criterion;
          if (err) {
            callback = once(callback);
            callback(err);
          } else if (++completed === size) {
            callback(null, sortByCriteria(array, criteria));
          }
        };
      }
    };
  }

  /**
   * @memberof async
   * @namespace sortBySeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBySeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3];
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBySeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [[1, 0], [3, 1], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBySeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortBySeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3]
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c']]
   * });
   *
   */
  function sortBySeries(collection, iterator, callback) {
    callback = onlyOnce(callback || noop);
    var size, key, value, keys, iter, item, array, criteria, iterate;
    var sync = false;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      array = collection;
      criteria = Array(size);
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      array = [];
      criteria = [];
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      array = Array(size);
      criteria = Array(size);
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null, []);
    }
    iterate();

    function arrayIterator() {
      value = collection[completed];
      iterator(value, done);
    }

    function arrayIteratorWithIndex() {
      value = collection[completed];
      iterator(value, completed, done);
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done) {
        return callback(null, sortByCriteria(array, criteria));
      }
      value = item.value;
      array[completed] = value;
      iterator(value, done);
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done) {
        return callback(null, sortByCriteria(array, criteria));
      }
      value = item.value;
      array[completed] = value;
      iterator(value, completed, done);
    }

    function objectIterator() {
      value = collection[keys[completed]];
      array[completed] = value;
      iterator(value, done);
    }

    function objectIteratorWithKey() {
      key = keys[completed];
      value = collection[key];
      array[completed] = value;
      iterator(value, key, done);
    }

    function done(err, criterion) {
      criteria[completed] = criterion;
      if (err) {
        callback(err);
      } else if (++completed === size) {
        iterate = throwError;
        callback(null, sortByCriteria(array, criteria));
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace sortByLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortByLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4, 5]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortByLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4, 5]
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortByLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4, 5]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.sortByLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4, 5]
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  function sortByLimit(collection, limit, iterator, callback) {
    callback = callback || noop;
    var size, index, key, value, array, keys, iter, item, criteria, iterate;
    var sync = false;
    var started = 0;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      array = collection;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      array = [];
      criteria = [];
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      array = Array(size);
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null, []);
    }
    criteria = criteria || Array(size);
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      if (started < size) {
        value = collection[started];
        iterator(value, createCallback(value, started++));
      }
    }

    function arrayIteratorWithIndex() {
      index = started++;
      if (index < size) {
        value = collection[index];
        iterator(value, index, createCallback(value, index));
      }
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done === false) {
        value = item.value;
        array[started] = value;
        iterator(value, createCallback(value, started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, sortByCriteria(array, criteria));
      }
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done === false) {
        value = item.value;
        array[started] = value;
        iterator(value, started, createCallback(value, started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, sortByCriteria(array, criteria));
      }
    }

    function objectIterator() {
      if (started < size) {
        value = collection[keys[started]];
        array[started] = value;
        iterator(value, createCallback(value, started++));
      }
    }

    function objectIteratorWithKey() {
      if (started < size) {
        key = keys[started];
        value = collection[key];
        array[started] = value;
        iterator(value, key, createCallback(value, started++));
      }
    }

    function createCallback(value, index) {
      var called = false;
      return function(err, criterion) {
        if (called) {
          throwError();
        }
        called = true;
        criteria[index] = criterion;
        if (err) {
          iterate = noop;
          callback(err);
          callback = noop;
        } else if (++completed === size) {
          callback(null, sortByCriteria(array, criteria));
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      };
    }
  }

  /**
   * @memberof async
   * @namespace some
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.some(array, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.some(array, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [[1, 0]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.some(object, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.some(object, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [[1, 'a']]
   * });
   *
   */
  function some(collection, iterator, callback) {
    callback = callback || noop;
    detect(collection, iterator, done);

    function done(err, res) {
      if (err) {
        return callback(err);
      }
      callback(null, !!res);
    }
  }

  /**
   * @memberof async
   * @namespace someSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someSeries(array, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someSeries(array, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [[1, 0]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someSeries(object, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someSeries(object, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [[1, 'a']]
   * });
   *
   */
  function someSeries(collection, iterator, callback) {
    callback = callback || noop;
    detectSeries(collection, iterator, done);

    function done(err, res) {
      if (err) {
        return callback(err);
      }
      callback(null, !!res);
    }
  }

  /**
   * @memberof async
   * @namespace someLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [[1, 0]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num % 2);
   *   }, num * 10);
   * };
   * async.someLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // true
   *   console.log(order); // [[1, 'a']]
   * });
   *
   */
  function someLimit(collection, limit, iterator, callback) {
    callback = callback || noop;
    detectLimit(collection, limit, iterator, done);

    function done(err, res) {
      if (err) {
        return callback(err);
      }
      callback(null, !!res);
    }
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   */
  function createEvery(arrayEach, baseEach, symbolEach) {
    var deny = createDetect(arrayEach, baseEach, symbolEach, false);

    return function every(collection, iterator, callback) {
      callback = callback || noop;
      deny(collection, iterator, done);

      function done(err, res) {
        if (err) {
          return callback(err);
        }
        callback(null, !res);
      }
    };
  }

  /**
   * @private
   */
  function createEverySeries() {
    var denySeries = createDetectSeries(false);

    return function everySeries(collection, iterator, callback) {
      callback = callback || noop;
      denySeries(collection, iterator, done);

      function done(err, res) {
        if (err) {
          return callback(err);
        }
        callback(null, !res);
      }
    };
  }

  /**
   * @private
   */
  function createEveryLimit() {
    var denyLimit = createDetectLimit(false);

    return function everyLimit(collection, limit, iterator, callback) {
      callback = callback || noop;
      denyLimit(collection, limit, iterator, done);

      function done(err, res) {
        if (err) {
          return callback(err);
        }
        callback(null, !res);
      }
    };
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   */
  function createConcat(arrayEach, baseEach, symbolEach) {
    return function concat(collection, iterator, callback) {
      callback = callback || noop;
      var size, result;
      var completed = 0;

      if (isArray(collection)) {
        size = collection.length;
        result = Array(size);
        arrayEach(collection, iterator, createCallback);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        result = [];
        size = symbolEach(collection, iterator, createCallback);
        size && size === completed && callback(null, result);
      } else if (typeof collection === obj) {
        var keys = nativeKeys(collection);
        size = keys.length;
        result = Array(size);
        baseEach(collection, iterator, createCallback, keys);
      }
      if (!size) {
        callback(null, []);
      }

      function createCallback(index) {
        return function done(err, res) {
          if (index === null) {
            throwError();
          }
          if (err) {
            index = null;
            callback = once(callback);
            arrayEachSync(result, function(array, index) {
              if (array === undefined) {
                result[index] = noop;
              }
            });
            callback(err, makeConcatResult(result));
            return;
          }
          switch (arguments.length) {
            case 0:
            case 1:
              result[index] = noop;
              break;
            case 2:
              result[index] = res;
              break;
            default:
              result[index] = slice(arguments, 1);
              break;
          }
          index = null;
          if (++completed === size) {
            callback(null, makeConcatResult(result));
          }
        };
      }
    };
  }

  /**
   * @memberof async
   * @namespace concatSeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concatSeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2];
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 3, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concatSeries(array, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [[1, 0], [3, 1], [2, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concatSeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [1, 3, 2]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 3, c: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concatSeries(object, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 2]
   *   console.log(order); // [[1, 'a'], [3, 'b'], [2, 'c']]
   * });
   *
   */
  function concatSeries(collection, iterator, callback) {
    callback = onlyOnce(callback || noop);
    var size, key, keys, iter, item, iterate;
    var sync = false;
    var result = [];
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null, result);
    }
    iterate();

    function arrayIterator() {
      iterator(collection[completed], done);
    }

    function arrayIteratorWithIndex() {
      iterator(collection[completed], completed, done);
    }

    function symbolIterator() {
      item = iter.next();
      item.done ? callback(null, result) : iterator(item.value, done);
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      item.done ? callback(null, result) : iterator(item.value, completed, done);
    }

    function objectIterator() {
      iterator(collection[keys[completed]], done);
    }

    function objectIteratorWithKey() {
      key = keys[completed];
      iterator(collection[key], key, done);
    }

    function done(err, array) {
      if (isArray(array)) {
        nativePush.apply(result, array);
      } else if (arguments.length >= 2) {
        nativePush.apply(result, slice(arguments, 1));
      }
      if (err) {
        callback(err, result);
      } else if (++completed === size) {
        iterate = throwError;
        callback(null, result);
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace concatLimit
   * @param {Array|Object} collection
   * @param {number} limit - limit >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concatLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 5, 2, 4]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1, 5, 3, 4, 2];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.cocnatLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 5, 2, 4]
   *   console.log(order); // [[1, 0], [3, 2], [5, 1], [2, 4], [4, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, [num]);
   *   }, num * 10);
   * };
   * async.concatLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 5, 2, 4]
   *   console.log(order); // [1, 3, 5, 2, 4]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1, b: 5, c: 3, d: 4, e: 2 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, num);
   *   }, num * 10);
   * };
   * async.cocnatLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // [1, 3, 5, 2, 4]
   *   console.log(order); // [[1, 'a'], [3, 'c'], [5, 'b'], [2, 'e'], [4, 'd']]
   * });
   *
   */
  function concatLimit(collection, limit, iterator, callback) {
    callback = callback || noop;
    var size, key, iter, item, iterate, result;
    var sync = false;
    var started = 0;
    var completed = 0;

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      result = [];
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      var keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null, []);
    }
    result = result || Array(size);
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      if (started < size) {
        iterator(collection[started], createCallback(started++));
      }
    }

    function arrayIteratorWithIndex() {
      if (started < size) {
        iterator(collection[started], started, createCallback(started++));
      }
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done === false) {
        iterator(item.value, createCallback(started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, makeConcatResult(result));
      }
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done === false) {
        iterator(item.value, started, createCallback(started++));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, makeConcatResult(result));
      }
    }

    function objectIterator() {
      if (started < size) {
        iterator(collection[keys[started]], createCallback(started++));
      }
    }

    function objectIteratorWithKey() {
      if (started < size) {
        key = keys[started];
        iterator(collection[key], key, createCallback(started++));
      }
    }

    function createCallback(index) {
      return function(err, res) {
        if (index === null) {
          throwError();
        }
        if (err) {
          index = null;
          iterate = noop;
          callback = once(callback);
          arrayEachSync(result, function(array, index) {
            if (array === undefined) {
              result[index] = noop;
            }
          });
          callback(err, makeConcatResult(result));
          return;
        }
        switch (arguments.length) {
          case 0:
          case 1:
            result[index] = noop;
            break;
          case 2:
            result[index] = res;
            break;
          default:
            result[index] = slice(arguments, 1);
            break;
        }
        index = null;
        if (++completed === size) {
          iterate = throwError;
          callback(null, makeConcatResult(result));
          callback = throwError;
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      };
    }
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   * @param {Function} symbolEach
   */
  function createGroupBy(arrayEach, baseEach, symbolEach) {
    return function groupBy(collection, iterator, callback) {
      callback = callback || noop;
      var size;
      var completed = 0;
      var result = {};

      if (isArray(collection)) {
        size = collection.length;
        arrayEach(collection, iterator, createCallback);
      } else if (!collection) {
      } else if (iteratorSymbol && collection[iteratorSymbol]) {
        size = symbolEach(collection, iterator, createCallback);
        size && size === completed && callback(null, result);
      } else if (typeof collection === obj) {
        var keys = nativeKeys(collection);
        size = keys.length;
        baseEach(collection, iterator, createCallback, keys);
      }
      if (!size) {
        callback(null, {});
      }

      function createCallback(value) {
        var called = false;
        return function done(err, key) {
          if (called) {
            throwError();
          }
          called = true;
          if (err) {
            callback = once(callback);
            callback(err, objectClone(result));
            return;
          }
          var array = result[key];
          if (!array) {
            result[key] = [value];
          } else {
            array.push(value);
          }
          if (++completed === size) {
            callback(null, result);
          }
        };
      }
    };
  }

  /**
   * @memberof async
   * @namespace groupBySeries
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [4.2, 6.4, 6.1];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBySeries(array, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.4, 6.1] }
   *   console.log(order); // [4.2, 6.4, 6.1]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [4.2, 6.4, 6.1];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBySeries(array, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.4, 6.1] }
   *   console.log(order); // [[4.2, 0], [6.4, 1], [6.1, 2]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 4.2, b: 6.4, c: 6.1 };
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBySeries(object, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.4, 6.1] }
   *   console.log(order); // [4.2, 6.4, 6.1]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 4.2, b: 6.4, c: 6.1 };
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupBySeries(object, iterator, function(err, res) {
   *   console.log(res); // { '4': [4.2], '6': [6.4, 6.1] }
   *   console.log(order); // [[4.2, 'a'], [6.4, 'b'], [6.1, 'c']]
   * });
   *
   */
  function groupBySeries(collection, iterator, callback) {
    callback = onlyOnce(callback || noop);
    var size, key, value, keys, iter, item, iterate;
    var sync = false;
    var completed = 0;
    var result = {};

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size) {
      return callback(null, result);
    }
    iterate();

    function arrayIterator() {
      value = collection[completed];
      iterator(value, done);
    }

    function arrayIteratorWithIndex() {
      value = collection[completed];
      iterator(value, completed, done);
    }

    function symbolIterator() {
      item = iter.next();
      value = item.value;
      item.done ? callback(null, result) : iterator(value, done);
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      value = item.value;
      item.done ? callback(null, result) : iterator(value, completed, done);
    }

    function objectIterator() {
      value = collection[keys[completed]];
      iterator(value, done);
    }

    function objectIteratorWithKey() {
      key = keys[completed];
      value = collection[key];
      iterator(value, key, done);
    }

    function done(err, key) {
      if (err) {
        iterate = throwError;
        callback = onlyOnce(callback);
        callback(err, objectClone(result));
        return;
      }
      var array = result[key];
      if (!array) {
        result[key] = [value];
      } else {
        array.push(value);
      }
      if (++completed === size) {
        iterate = throwError;
        callback(null, result);
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace groupByLimit
   * @param {Array|Object} collection
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * // array
   * var order = [];
   * var array = [1.1, 5.9, 3.2, 3.9, 2.1];
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupByLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '1': [1.1], '3': [3.2, 3.9], '5': [5.9], '2': [2.1] }
   *   console.log(order); // [1.1, 3.2, 5.9, 2.1, 3.9]
   * });
   *
   * @example
   *
   * // array with index
   * var order = [];
   * var array = [1.1, 5.9, 3.2, 3.9, 2.1];
   * var iterator = function(num, index, done) {
   *   setTimeout(function() {
   *     order.push([num, index]);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupByLimit(array, 2, iterator, function(err, res) {
   *   console.log(res); // { '1': [1.1], '3': [3.2, 3.9], '5': [5.9], '2': [2.1] }
   *   console.log(order); // [[1.1, 0], [3.2, 2], [5.9, 1], [2.1, 4], [3.9, 3]]
   * });
   *
   * @example
   *
   * // object
   * var order = [];
   * var object = { a: 1.1, b: 5.9, c: 3.2, d: 3.9, e: 2.1 }
   * var iterator = function(num, done) {
   *   setTimeout(function() {
   *     order.push(num);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupByLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { '1': [1.1], '3': [3.2, 3.9], '5': [5.9], '2': [2.1] }
   *   console.log(order); // [1.1, 3.2, 5.9, 2.1, 3.9]
   * });
   *
   * @example
   *
   * // object with key
   * var order = [];
   * var object = { a: 1.1, b: 5.9, c: 3.2, d: 3.9, e: 2.1 }
   * var iterator = function(num, key, done) {
   *   setTimeout(function() {
   *     order.push([num, key]);
   *     done(null, Math.floor(num));
   *   }, num * 10);
   * };
   * async.groupByLimit(object, 2, iterator, function(err, res) {
   *   console.log(res); // { '1': [1.1], '3': [3.2, 3.9], '5': [5.9], '2': [2.1] }
   *   console.log(order); // [[1.1, 'a'], [3.2, 'c'], [5.9, 'b'], [2.1, 'e'], [3.9, 'd']]
   * });
   *
   */
  function groupByLimit(collection, limit, iterator, callback) {
    callback = callback || noop;
    var size, index, key, value, keys, iter, item, iterate;
    var sync = false;
    var started = 0;
    var completed = 0;
    var result = {};

    if (isArray(collection)) {
      size = collection.length;
      iterate = iterator.length === 3 ? arrayIteratorWithIndex : arrayIterator;
    } else if (!collection) {
    } else if (iteratorSymbol && collection[iteratorSymbol]) {
      size = Infinity;
      iter = collection[iteratorSymbol]();
      iterate = iterator.length === 3 ? symbolIteratorWithKey : symbolIterator;
    } else if (typeof collection === obj) {
      keys = nativeKeys(collection);
      size = keys.length;
      iterate = iterator.length === 3 ? objectIteratorWithKey : objectIterator;
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null, result);
    }
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      if (started < size) {
        value = collection[started++];
        iterator(value, createCallback(value));
      }
    }

    function arrayIteratorWithIndex() {
      index = started++;
      if (index < size) {
        value = collection[index];
        iterator(value, index, createCallback(value));
      }
    }

    function symbolIterator() {
      item = iter.next();
      if (item.done === false) {
        started++;
        value = item.value;
        iterator(value, createCallback(value));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function symbolIteratorWithKey() {
      item = iter.next();
      if (item.done === false) {
        value = item.value;
        iterator(value, started++, createCallback(value));
      } else if (completed === started && iterator !== noop) {
        iterator = noop;
        callback(null, result);
      }
    }

    function objectIterator() {
      if (started < size) {
        value = collection[keys[started++]];
        iterator(value, createCallback(value));
      }
    }

    function objectIteratorWithKey() {
      if (started < size) {
        key = keys[started++];
        value = collection[key];
        iterator(value, key, createCallback(value));
      }
    }

    function createCallback(value) {
      var called = false;
      return function(err, key) {
        if (called) {
          throwError();
        }
        called = true;
        if (err) {
          iterate = noop;
          callback = once(callback);
          callback(err, objectClone(result));
          return;
        }
        var array = result[key];
        if (!array) {
          result[key] = [value];
        } else {
          array.push(value);
        }
        if (++completed === size) {
          callback(null, result);
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      };
    }
  }

  /**
   * @private
   * @param {Function} arrayEach
   * @param {Function} baseEach
   */
  function createParallel(arrayEach, baseEach) {
    return function parallel(tasks, callback) {
      callback = callback || noop;
      var size, keys, result;
      var completed = 0;

      if (isArray(tasks)) {
        size = tasks.length;
        result = Array(size);
        arrayEach(tasks, createCallback);
      } else if (tasks && typeof tasks === obj) {
        keys = nativeKeys(tasks);
        size = keys.length;
        result = {};
        baseEach(tasks, createCallback, keys);
      }
      if (!size) {
        callback(null, result);
      }

      function createCallback(key) {
        return function(err, res) {
          if (key === null) {
            throwError();
          }
          if (err) {
            key = null;
            callback = once(callback);
            callback(err, result);
            return;
          }
          result[key] = arguments.length <= 2 ? res : slice(arguments, 1);
          key = null;
          if (++completed === size) {
            callback(null, result);
          }
        };
      }
    };
  }

  /**
   * @memberof async
   * @namespace series
   * @param {Array|Object} tasks - functions
   * @param {Function} callback
   * @example
   *
   * var order = [];
   * var tasks = [
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(1);
   *      done(null, 1);
   *    }, 10);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(2);
   *      done(null, 2);
   *    }, 30);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(3);
   *      done(null, 3);
   *    }, 40);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(4);
   *      done(null, 4);
   *    }, 20);
   *  }
   * ];
   * async.series(tasks, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4];
   *   console.log(order); // [1, 2, 3, 4]
   * });
   *
   * @example
   *
   * var order = [];
   * var tasks = {
   *   'a': function(done) {
   *     setTimeout(function() {
   *       order.push(1);
   *       done(null, 1);
   *     }, 10);
   *   },
   *   'b': function(done) {
   *     setTimeout(function() {
   *       order.push(2);
   *       done(null, 2);
   *     }, 30);
   *   },
   *   'c': function(done) {
   *     setTimeout(function() {
   *       order.push(3);
   *       done(null, 3);
   *     }, 40);
   *   },
   *   'd': function(done) {
   *     setTimeout(function() {
   *       order.push(4);
   *       done(null, 4);
   *     }, 20);
   *   }
   * };
   * async.series(tasks, function(err, res) {
   *   console.log(res); // { a: 1, b: 2, c: 3, d:4 }
   *   console.log(order); // [1, 4, 2, 3]
   * });
   *
   */
  function series(tasks, callback) {
    callback = callback || noop;
    var size, key, keys, result, iterate;
    var sync = false;
    var completed = 0;

    if (isArray(tasks)) {
      size = tasks.length;
      result = Array(size);
      iterate = arrayIterator;
    } else if (tasks && typeof tasks === obj) {
      keys = nativeKeys(tasks);
      size = keys.length;
      result = {};
      iterate = objectIterator;
    } else {
      return callback(null);
    }
    if (!size) {
      return callback(null, result);
    }
    iterate();

    function arrayIterator() {
      key = completed;
      tasks[completed](done);
    }

    function objectIterator() {
      key = keys[completed];
      tasks[key](done);
    }

    function done(err, res) {
      if (err) {
        iterate = throwError;
        callback = onlyOnce(callback);
        callback(err, result);
        return;
      }
      result[key] = arguments.length <= 2 ? res : slice(arguments, 1);
      if (++completed === size) {
        iterate = throwError;
        callback(null, result);
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace parallelLimit
   * @param {Array|Object} tasks - functions
   * @param {number} limit - limit >= 1
   * @param {Function} callback
   * @example
   *
   * var order = [];
   * var tasks = [
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(1);
   *      done(null, 1);
   *    }, 10);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(2);
   *      done(null, 2);
   *    }, 50);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(3);
   *      done(null, 3);
   *    }, 30);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      order.push(4);
   *      done(null, 4);
   *    }, 40);
   *  }
   * ];
   * async.parallelLimit(tasks, 2, function(err, res) {
   *   console.log(res); // [1, 2, 3, 4];
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   * @example
   *
   * var order = [];
   * var tasks = {
   *   'a': function(done) {
   *     setTimeout(function() {
   *       order.push(1);
   *       done(null, 1);
   *     }, 10);
   *   },
   *   'b': function(done) {
   *     setTimeout(function() {
   *       order.push(2);
   *       done(null, 2);
   *     }, 50);
   *   },
   *   'c': function(done) {
   *     setTimeout(function() {
   *       order.push(3);
   *       done(null, 3);
   *     }, 20);
   *   },
   *   'd': function(done) {
   *     setTimeout(function() {
   *       order.push(4);
   *       done(null, 4);
   *     }, 40);
   *   }
   * };
   * async.parallelLimit(tasks, 2, function(err, res) {
   *   console.log(res); // { a: 1, b: 2, c: 3, d:4 }
   *   console.log(order); // [1, 3, 2, 4]
   * });
   *
   */
  function parallelLimit(tasks, limit, callback) {
    callback = callback || noop;
    var size, index, key, keys, result, iterate;
    var sync = false;
    var started = 0;
    var completed = 0;

    if (isArray(tasks)) {
      size = tasks.length;
      result = Array(size);
      iterate = arrayIterator;
    } else if (tasks && typeof tasks === obj) {
      keys = nativeKeys(tasks);
      size = keys.length;
      result = {};
      iterate = objectIterator;
    }
    if (!size || isNaN(limit) || limit < 1) {
      return callback(null, result);
    }
    timesSync(limit > size ? size : limit, iterate);

    function arrayIterator() {
      index = started++;
      if (index < size) {
        tasks[index](createCallback(index));
      }
    }

    function objectIterator() {
      if (started < size) {
        key = keys[started++];
        tasks[key](createCallback(key));
      }
    }

    function createCallback(key) {
      return function(err, res) {
        if (key === null) {
          throwError();
        }
        if (err) {
          key = null;
          iterate = noop;
          callback = once(callback);
          callback(err, result);
          return;
        }
        result[key] = arguments.length <= 2 ? res : slice(arguments, 1);
        key = null;
        if (++completed === size) {
          callback(null, result);
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      };
    }
  }

  /**
   * @memberof async
   * @namespace tryEach
   * @param {Array|Object} tasks - functions
   * @param {Function} callback
   * @example
   *
   * var tasks = [
   *  function(done) {
   *    setTimeout(function() {
   *      done(new Error('error'));
   *    }, 10);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      done(null, 2);
   *    }, 10);
   *  }
   * ];
   * async.tryEach(tasks, function(err, res) {
   *   console.log(res); // 2
   * });
   *
   * @example
   *
   * var tasks = [
   *  function(done) {
   *    setTimeout(function() {
   *      done(new Error('error1'));
   *    }, 10);
   *  },
   *  function(done) {
   *    setTimeout(function() {
   *      done(new Error('error2');
   *    }, 10);
   *  }
   * ];
   * async.tryEach(tasks, function(err, res) {
   *   console.log(err); // error2
   *   console.log(res); // undefined
   * });
   *
   */
  function tryEach(tasks, callback) {
    callback = callback || noop;
    var size, keys, iterate;
    var sync = false;
    var completed = 0;

    if (isArray(tasks)) {
      size = tasks.length;
      iterate = arrayIterator;
    } else if (tasks && typeof tasks === obj) {
      keys = nativeKeys(tasks);
      size = keys.length;
      iterate = objectIterator;
    }
    if (!size) {
      return callback(null);
    }
    iterate();

    function arrayIterator() {
      tasks[completed](done);
    }

    function objectIterator() {
      tasks[keys[completed]](done);
    }

    function done(err, res) {
      if (!err) {
        if (arguments.length <= 2) {
          callback(null, res);
        } else {
          callback(null, slice(arguments, 1));
        }
      } else if (++completed === size) {
        callback(err);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * check for waterfall tasks
   * @private
   * @param {Array} tasks
   * @param {Function} callback
   * @return {boolean}
   */
  function checkWaterfallTasks(tasks, callback) {
    if (!isArray(tasks)) {
      callback(new Error('First argument to waterfall must be an array of functions'));
      return false;
    }
    if (tasks.length === 0) {
      callback(null);
      return false;
    }
    return true;
  }

  /**
   * check for waterfall tasks
   * @private
   * @param {function} func
   * @param {Array|Object} args - arguments
   * @return {function} next
   */
  function waterfallIterator(func, args, next) {
    switch (args.length) {
      case 0:
      case 1:
        return func(next);
      case 2:
        return func(args[1], next);
      case 3:
        return func(args[1], args[2], next);
      case 4:
        return func(args[1], args[2], args[3], next);
      case 5:
        return func(args[1], args[2], args[3], args[4], next);
      case 6:
        return func(args[1], args[2], args[3], args[4], args[5], next);
      default:
        args = slice(args, 1);
        args.push(next);
        return func.apply(null, args);
    }
  }

  /**
   * @memberof async
   * @namespace waterfall
   * @param {Array} tasks - functions
   * @param {Function} callback
   * @example
   *
   * var order = [];
   * var tasks = [
   *   function(next) {
   *     setTimeout(function() {
   *       order.push(1);
   *       next(null, 1);
   *     }, 10);
   *   },
   *   function(arg1, next) {
   *     setTimeout(function() {
   *       order.push(2);
   *       next(null, 1, 2);
   *     }, 30);
   *   },
   *   function(arg1, arg2, next) {
   *     setTimeout(function() {
   *       order.push(3);
   *       next(null, 3);
   *     }, 20);
   *   },
   *   function(arg1, next) {
   *     setTimeout(function() {
   *       order.push(4);
   *       next(null, 1, 2, 3, 4);
   *     }, 40);
   *   }
   * ];
   * async.waterfall(tasks, function(err, arg1, arg2, arg3, arg4) {
   *   console.log(arg1, arg2, arg3, arg4); // 1 2 3 4
   * });
   *
   */
  function waterfall(tasks, callback) {
    callback = callback || noop;
    if (!checkWaterfallTasks(tasks, callback)) {
      return;
    }
    var func, args, done, sync;
    var completed = 0;
    var size = tasks.length;
    waterfallIterator(tasks[0], [], createCallback(0));

    function iterate() {
      waterfallIterator(func, args, createCallback(func));
    }

    function createCallback(index) {
      return function next(err, res) {
        if (index === undefined) {
          callback = noop;
          throwError();
        }
        index = undefined;
        if (err) {
          done = callback;
          callback = throwError;
          done(err);
          return;
        }
        if (++completed === size) {
          done = callback;
          callback = throwError;
          if (arguments.length <= 2) {
            done(err, res);
          } else {
            done.apply(null, createArray(arguments));
          }
          return;
        }
        if (sync) {
          args = arguments;
          func = tasks[completed] || throwError;
          nextTick(iterate);
        } else {
          sync = true;
          waterfallIterator(tasks[completed] || throwError, arguments, createCallback(completed));
        }
        sync = false;
      };
    }
  }

  /**
   * `angelFall` is like `waterfall` and inject callback to last argument of next task.
   *
   * @memberof async
   * @namespace angelFall
   * @param {Array} tasks - functions
   * @param {Function} callback
   * @example
   *
   * var order = [];
   * var tasks = [
   *   function(next) {
   *     setTimeout(function() {
   *       order.push(1);
   *       next(null, 1);
   *     }, 10);
   *   },
   *   function(arg1, empty, next) {
   *     setTimeout(function() {
   *       order.push(2);
   *       next(null, 1, 2);
   *     }, 30);
   *   },
   *   function(next) {
   *     setTimeout(function() {
   *       order.push(3);
   *       next(null, 3);
   *     }, 20);
   *   },
   *   function(arg1, empty1, empty2, empty3, next) {
   *     setTimeout(function() {
   *       order.push(4);
   *       next(null, 1, 2, 3, 4);
   *     }, 40);
   *   }
   * ];
   * async.angelFall(tasks, function(err, arg1, arg2, arg3, arg4) {
   *   console.log(arg1, arg2, arg3, arg4); // 1 2 3 4
   * });
   *
   */
  function angelFall(tasks, callback) {
    callback = callback || noop;
    if (!checkWaterfallTasks(tasks, callback)) {
      return;
    }
    var completed = 0;
    var sync = false;
    var size = tasks.length;
    var func = tasks[completed];
    var args = [];
    var iterate = function() {
      switch (func.length) {
        case 0:
          try {
            next(null, func());
          } catch (e) {
            next(e);
          }
          return;
        case 1:
          return func(next);
        case 2:
          return func(args[1], next);
        case 3:
          return func(args[1], args[2], next);
        case 4:
          return func(args[1], args[2], args[3], next);
        case 5:
          return func(args[1], args[2], args[3], args[4], next);
        default:
          args = slice(args, 1);
          args[func.length - 1] = next;
          return func.apply(null, args);
      }
    };
    iterate();

    function next(err, res) {
      if (err) {
        iterate = throwError;
        callback = onlyOnce(callback);
        callback(err);
        return;
      }
      if (++completed === size) {
        iterate = throwError;
        var done = callback;
        callback = throwError;
        if (arguments.length === 2) {
          done(err, res);
        } else {
          done.apply(null, createArray(arguments));
        }
        return;
      }
      func = tasks[completed];
      args = arguments;
      if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace whilst
   * @param {Function} test
   * @param {Function} iterator
   * @param {Function} callback
   */
  function whilst(test, iterator, callback) {
    callback = callback || noop;
    var sync = false;
    if (test()) {
      iterate();
    } else {
      callback(null);
    }

    function iterate() {
      if (sync) {
        nextTick(next);
      } else {
        sync = true;
        iterator(done);
      }
      sync = false;
    }

    function next() {
      iterator(done);
    }

    function done(err, arg) {
      if (err) {
        return callback(err);
      }
      if (arguments.length <= 2) {
        if (test(arg)) {
          iterate();
        } else {
          callback(null, arg);
        }
        return;
      }
      arg = slice(arguments, 1);
      if (test.apply(null, arg)) {
        iterate();
      } else {
        callback.apply(null, [null].concat(arg));
      }
    }
  }

  /**
   * @memberof async
   * @namespace doWhilst
   * @param {Function} iterator
   * @param {Function} test
   * @param {Function} callback
   */
  function doWhilst(iterator, test, callback) {
    callback = callback || noop;
    var sync = false;
    next();

    function iterate() {
      if (sync) {
        nextTick(next);
      } else {
        sync = true;
        iterator(done);
      }
      sync = false;
    }

    function next() {
      iterator(done);
    }

    function done(err, arg) {
      if (err) {
        return callback(err);
      }
      if (arguments.length <= 2) {
        if (test(arg)) {
          iterate();
        } else {
          callback(null, arg);
        }
        return;
      }
      arg = slice(arguments, 1);
      if (test.apply(null, arg)) {
        iterate();
      } else {
        callback.apply(null, [null].concat(arg));
      }
    }
  }

  /**
   * @memberof async
   * @namespace until
   * @param {Function} test
   * @param {Function} iterator
   * @param {Function} callback
   */
  function until(test, iterator, callback) {
    callback = callback || noop;
    var sync = false;
    if (!test()) {
      iterate();
    } else {
      callback(null);
    }

    function iterate() {
      if (sync) {
        nextTick(next);
      } else {
        sync = true;
        iterator(done);
      }
      sync = false;
    }

    function next() {
      iterator(done);
    }

    function done(err, arg) {
      if (err) {
        return callback(err);
      }
      if (arguments.length <= 2) {
        if (!test(arg)) {
          iterate();
        } else {
          callback(null, arg);
        }
        return;
      }
      arg = slice(arguments, 1);
      if (!test.apply(null, arg)) {
        iterate();
      } else {
        callback.apply(null, [null].concat(arg));
      }
    }
  }

  /**
   * @memberof async
   * @namespace doUntil
   * @param {Function} iterator
   * @param {Function} test
   * @param {Function} callback
   */
  function doUntil(iterator, test, callback) {
    callback = callback || noop;
    var sync = false;
    next();

    function iterate() {
      if (sync) {
        nextTick(next);
      } else {
        sync = true;
        iterator(done);
      }
      sync = false;
    }

    function next() {
      iterator(done);
    }

    function done(err, arg) {
      if (err) {
        return callback(err);
      }
      if (arguments.length <= 2) {
        if (!test(arg)) {
          iterate();
        } else {
          callback(null, arg);
        }
        return;
      }
      arg = slice(arguments, 1);
      if (!test.apply(null, arg)) {
        iterate();
      } else {
        callback.apply(null, [null].concat(arg));
      }
    }
  }

  /**
   * @memberof async
   * @namespace during
   * @param {Function} test
   * @param {Function} iterator
   * @param {Function} callback
   */
  function during(test, iterator, callback) {
    callback = callback || noop;
    _test();

    function _test() {
      test(iterate);
    }

    function iterate(err, truth) {
      if (err) {
        return callback(err);
      }
      if (truth) {
        iterator(done);
      } else {
        callback(null);
      }
    }

    function done(err) {
      if (err) {
        return callback(err);
      }
      _test();
    }
  }

  /**
   * @memberof async
   * @namespace doDuring
   * @param {Function} test
   * @param {Function} iterator
   * @param {Function} callback
   */
  function doDuring(iterator, test, callback) {
    callback = callback || noop;
    iterate(null, true);

    function iterate(err, truth) {
      if (err) {
        return callback(err);
      }
      if (truth) {
        iterator(done);
      } else {
        callback(null);
      }
    }

    function done(err, res) {
      if (err) {
        return callback(err);
      }
      switch (arguments.length) {
        case 0:
        case 1:
          test(iterate);
          break;
        case 2:
          test(res, iterate);
          break;
        default:
          var args = slice(arguments, 1);
          args.push(iterate);
          test.apply(null, args);
          break;
      }
    }
  }

  /**
   * @memberof async
   * @namespace forever
   */
  function forever(iterator, callback) {
    var sync = false;
    iterate();

    function iterate() {
      iterator(next);
    }

    function next(err) {
      if (err) {
        if (callback) {
          return callback(err);
        }
        throw err;
      }
      if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace compose
   */
  function compose() {
    return seq.apply(null, reverse(arguments));
  }

  /**
   * @memberof async
   * @namespace seq
   */
  function seq(/* functions... */) {
    var fns = createArray(arguments);

    return function() {
      var self = this;
      var args = createArray(arguments);
      var callback = args[args.length - 1];
      if (typeof callback === func) {
        args.pop();
      } else {
        callback = noop;
      }
      reduce(fns, args, iterator, done);

      function iterator(newargs, fn, callback) {
        var func = function(err) {
          var nextargs = slice(arguments, 1);
          callback(err, nextargs);
        };
        newargs.push(func);
        fn.apply(self, newargs);
      }

      function done(err, res) {
        res = isArray(res) ? res : [res];
        res.unshift(err);
        callback.apply(self, res);
      }
    };
  }

  function createApplyEach(func) {
    return function applyEach(fns /* arguments */) {
      var go = function() {
        var self = this;
        var args = createArray(arguments);
        var callback = args.pop() || noop;
        return func(fns, iterator, callback);

        function iterator(fn, done) {
          fn.apply(self, args.concat([done]));
        }
      };
      if (arguments.length > 1) {
        var args = slice(arguments, 1);
        return go.apply(this, args);
      } else {
        return go;
      }
    };
  }

  /**
   * @see https://github.com/caolan/async/blob/master/lib/internal/DoublyLinkedList.js
   */
  function DLL() {
    this.head = null;
    this.tail = null;
    this.length = 0;
  }

  DLL.prototype._removeLink = function(node) {
    var prev = node.prev;
    var next = node.next;
    if (prev) {
      prev.next = next;
    } else {
      this.head = next;
    }
    if (next) {
      next.prev = prev;
    } else {
      this.tail = prev;
    }
    node.prev = null;
    node.next = null;
    this.length--;
    return node;
  };

  DLL.prototype.empty = DLL;

  DLL.prototype._setInitial = function(node) {
    this.length = 1;
    this.head = this.tail = node;
  };

  DLL.prototype.insertBefore = function(node, newNode) {
    newNode.prev = node.prev;
    newNode.next = node;
    if (node.prev) {
      node.prev.next = newNode;
    } else {
      this.head = newNode;
    }
    node.prev = newNode;
    this.length++;
  };

  DLL.prototype.unshift = function(node) {
    if (this.head) {
      this.insertBefore(this.head, node);
    } else {
      this._setInitial(node);
    }
  };

  DLL.prototype.push = function(node) {
    var tail = this.tail;
    if (tail) {
      node.prev = tail;
      node.next = tail.next;
      this.tail = node;
      tail.next = node;
      this.length++;
    } else {
      this._setInitial(node);
    }
  };

  DLL.prototype.shift = function() {
    return this.head && this._removeLink(this.head);
  };

  DLL.prototype.splice = function(end) {
    var task;
    var tasks = [];
    while (end-- && (task = this.shift())) {
      tasks.push(task);
    }
    return tasks;
  };

  DLL.prototype.remove = function(test) {
    var node = this.head;
    while (node) {
      if (test(node)) {
        this._removeLink(node);
      }
      node = node.next;
    }
    return this;
  };

  /**
   * @private
   */
  function baseQueue(isQueue, worker, concurrency, payload) {
    if (concurrency === undefined) {
      concurrency = 1;
    } else if (isNaN(concurrency) || concurrency < 1) {
      throw new Error('Concurrency must not be zero');
    }

    var workers = 0;
    var workersList = [];
    var _callback, _unshift;

    var q = {
      _tasks: new DLL(),
      concurrency: concurrency,
      payload: payload,
      saturated: noop,
      unsaturated: noop,
      buffer: concurrency / 4,
      empty: noop,
      drain: noop,
      error: noop,
      started: false,
      paused: false,
      push: push,
      kill: kill,
      unshift: unshift,
      remove: remove,
      process: isQueue ? runQueue : runCargo,
      length: getLength,
      running: running,
      workersList: getWorkersList,
      idle: idle,
      pause: pause,
      resume: resume,
      _worker: worker
    };
    return q;

    function push(tasks, callback) {
      _insert(tasks, callback);
    }

    function unshift(tasks, callback) {
      _insert(tasks, callback, true);
    }

    function _exec(task) {
      var item = {
        data: task,
        callback: _callback
      };
      if (_unshift) {
        q._tasks.unshift(item);
      } else {
        q._tasks.push(item);
      }
      nextTick(q.process);
    }

    function _insert(tasks, callback, unshift) {
      if (callback == null) {
        callback = noop;
      } else if (typeof callback !== 'function') {
        throw new Error('task callback must be a function');
      }
      q.started = true;
      var _tasks = isArray(tasks) ? tasks : [tasks];

      if (tasks === undefined || !_tasks.length) {
        if (q.idle()) {
          nextTick(q.drain);
        }
        return;
      }

      _unshift = unshift;
      _callback = callback;
      arrayEachSync(_tasks, _exec);
    }

    function kill() {
      q.drain = noop;
      q._tasks.empty();
    }

    function _next(q, tasks) {
      var called = false;
      return function done(err, res) {
        if (called) {
          throwError();
        }
        called = true;

        workers--;
        var task;
        var index = -1;
        var size = workersList.length;
        var taskIndex = -1;
        var taskSize = tasks.length;
        var useApply = arguments.length > 2;
        var args = useApply && createArray(arguments);
        while (++taskIndex < taskSize) {
          task = tasks[taskIndex];
          while (++index < size) {
            if (workersList[index] === task) {
              if (index === 0) {
                workersList.shift();
              } else {
                workersList.splice(index, 1);
              }
              index = size;
              size--;
            }
          }
          index = -1;
          if (useApply) {
            task.callback.apply(task, args);
          } else {
            task.callback(err, res);
          }
          if (err) {
            q.error(err, task.data);
          }
        }

        if (workers <= q.concurrency - q.buffer) {
          q.unsaturated();
        }

        if (q._tasks.length + workers === 0) {
          q.drain();
        }
        q.process();
      };
    }

    function runQueue() {
      while (!q.paused && workers < q.concurrency && q._tasks.length) {
        var task = q._tasks.shift();
        workers++;
        workersList.push(task);
        if (q._tasks.length === 0) {
          q.empty();
        }
        if (workers === q.concurrency) {
          q.saturated();
        }
        var done = _next(q, [task]);
        worker(task.data, done);
      }
    }

    function runCargo() {
      while (!q.paused && workers < q.concurrency && q._tasks.length) {
        var tasks = q._tasks.splice(q.payload || q._tasks.length);
        var index = -1;
        var size = tasks.length;
        var data = Array(size);
        while (++index < size) {
          data[index] = tasks[index].data;
        }
        workers++;
        nativePush.apply(workersList, tasks);
        if (q._tasks.length === 0) {
          q.empty();
        }
        if (workers === q.concurrency) {
          q.saturated();
        }
        var done = _next(q, tasks);
        worker(data, done);
      }
    }

    function getLength() {
      return q._tasks.length;
    }

    function running() {
      return workers;
    }

    function getWorkersList() {
      return workersList;
    }

    function idle() {
      return q.length() + workers === 0;
    }

    function pause() {
      q.paused = true;
    }

    function _resume() {
      nextTick(q.process);
    }

    function resume() {
      if (q.paused === false) {
        return;
      }
      q.paused = false;
      var count = q.concurrency < q._tasks.length ? q.concurrency : q._tasks.length;
      timesSync(count, _resume);
    }

    /**
     * @param {Function} test
     */
    function remove(test) {
      q._tasks.remove(test);
    }
  }

  /**
   * @memberof async
   * @namespace queue
   */
  function queue(worker, concurrency) {
    return baseQueue(true, worker, concurrency);
  }

  /**
   * @memberof async
   * @namespace priorityQueue
   */
  function priorityQueue(worker, concurrency) {
    var q = baseQueue(true, worker, concurrency);
    q.push = push;
    delete q.unshift;
    return q;

    function push(tasks, priority, callback) {
      q.started = true;
      priority = priority || 0;
      var _tasks = isArray(tasks) ? tasks : [tasks];
      var taskSize = _tasks.length;

      if (tasks === undefined || taskSize === 0) {
        if (q.idle()) {
          nextTick(q.drain);
        }
        return;
      }

      callback = typeof callback === func ? callback : noop;
      var nextNode = q._tasks.head;
      while (nextNode && priority >= nextNode.priority) {
        nextNode = nextNode.next;
      }
      while (taskSize--) {
        var item = {
          data: _tasks[taskSize],
          priority: priority,
          callback: callback
        };
        if (nextNode) {
          q._tasks.insertBefore(nextNode, item);
        } else {
          q._tasks.push(item);
        }
        nextTick(q.process);
      }
    }
  }

  /**
   * @memberof async
   * @namespace cargo
   */
  function cargo(worker, payload) {
    return baseQueue(false, worker, 1, payload);
  }

  /**
   * @memberof async
   * @namespace auto
   * @param {Object} tasks
   * @param {number} [concurrency]
   * @param {Function} [callback]
   */
  function auto(tasks, concurrency, callback) {
    if (typeof concurrency === func) {
      callback = concurrency;
      concurrency = null;
    }
    var keys = nativeKeys(tasks);
    var rest = keys.length;
    var results = {};
    if (rest === 0) {
      return callback(null, results);
    }
    var runningTasks = 0;
    var readyTasks = [];
    var listeners = Object.create(null);
    callback = onlyOnce(callback || noop);
    concurrency = concurrency || rest;

    baseEachSync(tasks, iterator, keys);
    proceedQueue();

    function iterator(task, key) {
      // no dependencies
      var _task, _taskSize;
      if (!isArray(task)) {
        _task = task;
        _taskSize = 0;
        readyTasks.push([_task, _taskSize, done]);
        return;
      }
      var dependencySize = task.length - 1;
      _task = task[dependencySize];
      _taskSize = dependencySize;
      if (dependencySize === 0) {
        readyTasks.push([_task, _taskSize, done]);
        return;
      }
      // dependencies
      var index = -1;
      while (++index < dependencySize) {
        var dependencyName = task[index];
        if (notInclude(keys, dependencyName)) {
          var msg =
            'async.auto task `' +
            key +
            '` has non-existent dependency `' +
            dependencyName +
            '` in ' +
            task.join(', ');
          throw new Error(msg);
        }
        var taskListeners = listeners[dependencyName];
        if (!taskListeners) {
          taskListeners = listeners[dependencyName] = [];
        }
        taskListeners.push(taskListener);
      }

      function done(err, arg) {
        if (key === null) {
          throwError();
        }
        arg = arguments.length <= 2 ? arg : slice(arguments, 1);
        if (err) {
          rest = 0;
          runningTasks = 0;
          readyTasks.length = 0;
          var safeResults = objectClone(results);
          safeResults[key] = arg;
          key = null;
          var _callback = callback;
          callback = noop;
          _callback(err, safeResults);
          return;
        }
        runningTasks--;
        rest--;
        results[key] = arg;
        taskComplete(key);
        key = null;
      }

      function taskListener() {
        if (--dependencySize === 0) {
          readyTasks.push([_task, _taskSize, done]);
        }
      }
    }

    function proceedQueue() {
      if (readyTasks.length === 0 && runningTasks === 0) {
        if (rest !== 0) {
          throw new Error('async.auto task has cyclic dependencies');
        }
        return callback(null, results);
      }
      while (readyTasks.length && runningTasks < concurrency && callback !== noop) {
        runningTasks++;
        var array = readyTasks.shift();
        if (array[1] === 0) {
          array[0](array[2]);
        } else {
          array[0](results, array[2]);
        }
      }
    }

    function taskComplete(key) {
      var taskListeners = listeners[key] || [];
      arrayEachSync(taskListeners, function(task) {
        task();
      });
      proceedQueue();
    }
  }

  var FN_ARGS = /^(function)?\s*[^\(]*\(\s*([^\)]*)\)/m;
  var FN_ARG_SPLIT = /,/;
  var FN_ARG = /(=.+)?(\s*)$/;
  var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/gm;

  /**
   * parse function arguments for `autoInject`
   *
   * @private
   */
  function parseParams(func) {
    func = func.toString().replace(STRIP_COMMENTS, '');
    func = func.match(FN_ARGS)[2].replace(' ', '');
    func = func ? func.split(FN_ARG_SPLIT) : [];
    func = func.map(function(arg) {
      return arg.replace(FN_ARG, '').trim();
    });
    return func;
  }

  /**
   * @memberof async
   * @namespace autoInject
   * @param {Object} tasks
   * @param {number} [concurrency]
   * @param {Function} [callback]
   */
  function autoInject(tasks, concurrency, callback) {
    var newTasks = {};
    baseEachSync(tasks, iterator, nativeKeys(tasks));
    auto(newTasks, concurrency, callback);

    function iterator(task, key) {
      var params;
      var taskLength = task.length;

      if (isArray(task)) {
        if (taskLength === 0) {
          throw new Error('autoInject task functions require explicit parameters.');
        }
        params = createArray(task);
        taskLength = params.length - 1;
        task = params[taskLength];
        if (taskLength === 0) {
          newTasks[key] = task;
          return;
        }
      } else if (taskLength === 1) {
        newTasks[key] = task;
        return;
      } else {
        params = parseParams(task);
        if (taskLength === 0 && params.length === 0) {
          throw new Error('autoInject task functions require explicit parameters.');
        }
        taskLength = params.length - 1;
      }
      params[taskLength] = newTask;
      newTasks[key] = params;

      function newTask(results, done) {
        switch (taskLength) {
          case 1:
            task(results[params[0]], done);
            break;
          case 2:
            task(results[params[0]], results[params[1]], done);
            break;
          case 3:
            task(results[params[0]], results[params[1]], results[params[2]], done);
            break;
          default:
            var i = -1;
            while (++i < taskLength) {
              params[i] = results[params[i]];
            }
            params[i] = done;
            task.apply(null, params);
            break;
        }
      }
    }
  }

  /**
   * @memberof async
   * @namespace retry
   * @param {integer|Object|Function} opts
   * @param {Function} [task]
   * @param {Function} [callback]
   */
  function retry(opts, task, callback) {
    var times, intervalFunc, errorFilter;
    var count = 0;
    if (arguments.length < 3 && typeof opts === func) {
      callback = task || noop;
      task = opts;
      opts = null;
      times = DEFAULT_TIMES;
    } else {
      callback = callback || noop;
      switch (typeof opts) {
        case 'object':
          if (typeof opts.errorFilter === func) {
            errorFilter = opts.errorFilter;
          }
          var interval = opts.interval;
          switch (typeof interval) {
            case func:
              intervalFunc = interval;
              break;
            case 'string':
            case 'number':
              interval = +interval;
              intervalFunc = interval
                ? function() {
                    return interval;
                  }
                : function() {
                    return DEFAULT_INTERVAL;
                  };
              break;
          }
          times = +opts.times || DEFAULT_TIMES;
          break;
        case 'number':
          times = opts || DEFAULT_TIMES;
          break;
        case 'string':
          times = +opts || DEFAULT_TIMES;
          break;
        default:
          throw new Error('Invalid arguments for async.retry');
      }
    }
    if (typeof task !== 'function') {
      throw new Error('Invalid arguments for async.retry');
    }

    if (intervalFunc) {
      task(intervalCallback);
    } else {
      task(simpleCallback);
    }

    function simpleIterator() {
      task(simpleCallback);
    }

    function simpleCallback(err, res) {
      if (++count === times || !err || (errorFilter && !errorFilter(err))) {
        if (arguments.length <= 2) {
          return callback(err, res);
        }
        var args = createArray(arguments);
        return callback.apply(null, args);
      }
      simpleIterator();
    }

    function intervalIterator() {
      task(intervalCallback);
    }

    function intervalCallback(err, res) {
      if (++count === times || !err || (errorFilter && !errorFilter(err))) {
        if (arguments.length <= 2) {
          return callback(err, res);
        }
        var args = createArray(arguments);
        return callback.apply(null, args);
      }
      setTimeout(intervalIterator, intervalFunc(count));
    }
  }

  function retryable(opts, task) {
    if (!task) {
      task = opts;
      opts = null;
    }
    return done;

    function done() {
      var taskFn;
      var args = createArray(arguments);
      var lastIndex = args.length - 1;
      var callback = args[lastIndex];
      switch (task.length) {
        case 1:
          taskFn = task1;
          break;
        case 2:
          taskFn = task2;
          break;
        case 3:
          taskFn = task3;
          break;
        default:
          taskFn = task4;
      }
      if (opts) {
        retry(opts, taskFn, callback);
      } else {
        retry(taskFn, callback);
      }

      function task1(done) {
        task(done);
      }

      function task2(done) {
        task(args[0], done);
      }

      function task3(done) {
        task(args[0], args[1], done);
      }

      function task4(callback) {
        args[lastIndex] = callback;
        task.apply(null, args);
      }
    }
  }

  /**
   * @memberof async
   * @namespace iterator
   */
  function iterator(tasks) {
    var size = 0;
    var keys = [];
    if (isArray(tasks)) {
      size = tasks.length;
    } else {
      keys = nativeKeys(tasks);
      size = keys.length;
    }
    return makeCallback(0);

    function makeCallback(index) {
      var fn = function() {
        if (size) {
          var key = keys[index] || index;
          tasks[key].apply(null, createArray(arguments));
        }
        return fn.next();
      };
      fn.next = function() {
        return index < size - 1 ? makeCallback(index + 1) : null;
      };
      return fn;
    }
  }

  /**
   * @memberof async
   * @namespace apply
   */
  function apply(func) {
    switch (arguments.length) {
      case 0:
      case 1:
        return func;
      case 2:
        return func.bind(null, arguments[1]);
      case 3:
        return func.bind(null, arguments[1], arguments[2]);
      case 4:
        return func.bind(null, arguments[1], arguments[2], arguments[3]);
      case 5:
        return func.bind(null, arguments[1], arguments[2], arguments[3], arguments[4]);
      default:
        var size = arguments.length;
        var index = 0;
        var args = Array(size);
        args[index] = null;
        while (++index < size) {
          args[index] = arguments[index];
        }
        return func.bind.apply(func, args);
    }
  }

  /**
   * @memberof async
   * @namespace timeout
   * @param {Function} func
   * @param {number} millisec
   * @param {*} info
   */
  function timeout(func, millisec, info) {
    var callback, timer;
    return wrappedFunc;

    function wrappedFunc() {
      timer = setTimeout(timeoutCallback, millisec);
      var args = createArray(arguments);
      var lastIndex = args.length - 1;
      callback = args[lastIndex];
      args[lastIndex] = injectedCallback;
      simpleApply(func, args);
    }

    function timeoutCallback() {
      var name = func.name || 'anonymous';
      var err = new Error('Callback function "' + name + '" timed out.');
      err.code = 'ETIMEDOUT';
      if (info) {
        err.info = info;
      }
      timer = null;
      callback(err);
    }

    function injectedCallback() {
      if (timer !== null) {
        simpleApply(callback, createArray(arguments));
        clearTimeout(timer);
      }
    }

    function simpleApply(func, args) {
      switch (args.length) {
        case 0:
          func();
          break;
        case 1:
          func(args[0]);
          break;
        case 2:
          func(args[0], args[1]);
          break;
        default:
          func.apply(null, args);
          break;
      }
    }
  }

  /**
   * @memberof async
   * @namespace times
   * @param {number} n - n >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * var iterator = function(n, done) {
   *   done(null, n);
   * };
   * async.times(4, iterator, function(err, res) {
   *   console.log(res); // [0, 1, 2, 3];
   * });
   *
   */
  function times(n, iterator, callback) {
    callback = callback || noop;
    n = +n;
    if (isNaN(n) || n < 1) {
      return callback(null, []);
    }
    var result = Array(n);
    timesSync(n, iterate);

    function iterate(num) {
      iterator(num, createCallback(num));
    }

    function createCallback(index) {
      return function(err, res) {
        if (index === null) {
          throwError();
        }
        result[index] = res;
        index = null;
        if (err) {
          callback(err);
          callback = noop;
        } else if (--n === 0) {
          callback(null, result);
        }
      };
    }
  }

  /**
   * @memberof async
   * @namespace timesSeries
   * @param {number} n - n >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * var iterator = function(n, done) {
   *   done(null, n);
   * };
   * async.timesSeries(4, iterator, function(err, res) {
   *   console.log(res); // [0, 1, 2, 3];
   * });
   *
   */
  function timesSeries(n, iterator, callback) {
    callback = callback || noop;
    n = +n;
    if (isNaN(n) || n < 1) {
      return callback(null, []);
    }
    var result = Array(n);
    var sync = false;
    var completed = 0;
    iterate();

    function iterate() {
      iterator(completed, done);
    }

    function done(err, res) {
      result[completed] = res;
      if (err) {
        callback(err);
        callback = throwError;
      } else if (++completed >= n) {
        callback(null, result);
        callback = throwError;
      } else if (sync) {
        nextTick(iterate);
      } else {
        sync = true;
        iterate();
      }
      sync = false;
    }
  }

  /**
   * @memberof async
   * @namespace timesLimit
   * @param {number} n - n >= 1
   * @param {number} limit - n >= 1
   * @param {Function} iterator
   * @param {Function} callback
   * @example
   *
   * var iterator = function(n, done) {
   *   done(null, n);
   * };
   * async.timesLimit(4, 2, iterator, function(err, res) {
   *   console.log(res); // [0, 1, 2, 3];
   * });
   *
   */
  function timesLimit(n, limit, iterator, callback) {
    callback = callback || noop;
    n = +n;
    if (isNaN(n) || n < 1 || isNaN(limit) || limit < 1) {
      return callback(null, []);
    }
    var result = Array(n);
    var sync = false;
    var started = 0;
    var completed = 0;
    timesSync(limit > n ? n : limit, iterate);

    function iterate() {
      var index = started++;
      if (index < n) {
        iterator(index, createCallback(index));
      }
    }

    function createCallback(index) {
      return function(err, res) {
        if (index === null) {
          throwError();
        }
        result[index] = res;
        index = null;
        if (err) {
          callback(err);
          callback = noop;
        } else if (++completed >= n) {
          callback(null, result);
          callback = throwError;
        } else if (sync) {
          nextTick(iterate);
        } else {
          sync = true;
          iterate();
        }
        sync = false;
      };
    }
  }

  /**
   * @memberof async
   * @namespace race
   * @param {Array|Object} tasks - functions
   * @param {Function} callback
   * @example
   *
   * // array
   * var called = 0;
   * var tasks = [
   *   function(done) {
   *     setTimeout(function() {
   *       called++;
   *       done(null, '1');
   *     }, 30);
   *   },
   *   function(done) {
   *     setTimeout(function() {
   *       called++;
   *       done(null, '2');
   *     }, 20);
   *   },
   *   function(done) {
   *     setTimeout(function() {
   *       called++;
   *       done(null, '3');
   *     }, 10);
   *   }
   * ];
   * async.race(tasks, function(err, res) {
   *   console.log(res); // '3'
   *   console.log(called); // 1
   *   setTimeout(function() {
   *     console.log(called); // 3
   *   }, 50);
   * });
   *
   * @example
   *
   * // object
   * var called = 0;
   * var tasks = {
   *   'test1': function(done) {
   *     setTimeout(function() {
   *       called++;
   *       done(null, '1');
   *     }, 30);
   *   },
   *   'test2': function(done) {
   *     setTimeout(function() {
   *       called++;
   *       done(null, '2');
   *     }, 20);
   *   },
   *   'test3': function(done) {
   *     setTimeout(function() {
   *       called++;
   *       done(null, '3');
   *     }, 10);
   *   }
   * };
   * async.race(tasks, function(err, res) {
   *   console.log(res); // '3'
   *   console.log(called); // 1
   *   setTimeout(function() {
   *     console.log(called); // 3
   *     done();
   *   }, 50);
   * });
   *
   */
  function race(tasks, callback) {
    callback = once(callback || noop);
    var size, keys;
    var index = -1;
    if (isArray(tasks)) {
      size = tasks.length;
      while (++index < size) {
        tasks[index](callback);
      }
    } else if (tasks && typeof tasks === obj) {
      keys = nativeKeys(tasks);
      size = keys.length;
      while (++index < size) {
        tasks[keys[index]](callback);
      }
    } else {
      return callback(new TypeError('First argument to race must be a collection of functions'));
    }
    if (!size) {
      callback(null);
    }
  }

  /**
   * @memberof async
   * @namespace memoize
   */
  function memoize(fn, hasher) {
    hasher =
      hasher ||
      function(hash) {
        return hash;
      };

    var memo = {};
    var queues = {};
    var memoized = function() {
      var args = createArray(arguments);
      var callback = args.pop();
      var key = hasher.apply(null, args);
      if (has(memo, key)) {
        nextTick(function() {
          callback.apply(null, memo[key]);
        });
        return;
      }
      if (has(queues, key)) {
        return queues[key].push(callback);
      }

      queues[key] = [callback];
      args.push(done);
      fn.apply(null, args);

      function done(err) {
        var args = createArray(arguments);
        if (!err) {
          memo[key] = args;
        }
        var q = queues[key];
        delete queues[key];

        var i = -1;
        var size = q.length;
        while (++i < size) {
          q[i].apply(null, args);
        }
      }
    };
    memoized.memo = memo;
    memoized.unmemoized = fn;
    return memoized;
  }

  /**
   * @memberof async
   * @namespace unmemoize
   */
  function unmemoize(fn) {
    return function() {
      return (fn.unmemoized || fn).apply(null, arguments);
    };
  }

  /**
   * @memberof async
   * @namespace ensureAsync
   */
  function ensureAsync(fn) {
    return function(/* ...args, callback */) {
      var args = createArray(arguments);
      var lastIndex = args.length - 1;
      var callback = args[lastIndex];
      var sync = true;
      args[lastIndex] = done;
      fn.apply(this, args);
      sync = false;

      function done() {
        var innerArgs = createArray(arguments);
        if (sync) {
          nextTick(function() {
            callback.apply(null, innerArgs);
          });
        } else {
          callback.apply(null, innerArgs);
        }
      }
    };
  }

  /**
   * @memberof async
   * @namespace constant
   */
  function constant(/* values... */) {
    var args = [null].concat(createArray(arguments));
    return function(callback) {
      callback = arguments[arguments.length - 1];
      callback.apply(this, args);
    };
  }

  function asyncify(fn) {
    return function(/* args..., callback */) {
      var args = createArray(arguments);
      var callback = args.pop();
      var result;
      try {
        result = fn.apply(this, args);
      } catch (e) {
        return callback(e);
      }
      if (result && typeof result.then === func) {
        result.then(
          function(value) {
            invokeCallback(callback, null, value);
          },
          function(err) {
            invokeCallback(callback, err && err.message ? err : new Error(err));
          }
        );
      } else {
        callback(null, result);
      }
    };
  }

  function invokeCallback(callback, err, value) {
    try {
      callback(err, value);
    } catch (e) {
      nextTick(rethrow, e);
    }
  }

  function rethrow(error) {
    throw error;
  }

  /**
   * @memberof async
   * @namespace reflect
   * @param {Function} func
   * @return {Function}
   */
  function reflect(func) {
    return function(/* args..., callback */) {
      var callback;
      switch (arguments.length) {
        case 1:
          callback = arguments[0];
          return func(done);
        case 2:
          callback = arguments[1];
          return func(arguments[0], done);
        default:
          var args = createArray(arguments);
          var lastIndex = args.length - 1;
          callback = args[lastIndex];
          args[lastIndex] = done;
          func.apply(this, args);
      }

      function done(err, res) {
        if (err) {
          return callback(null, {
            error: err
          });
        }
        if (arguments.length > 2) {
          res = slice(arguments, 1);
        }
        callback(null, {
          value: res
        });
      }
    };
  }

  /**
   * @memberof async
   * @namespace reflectAll
   * @param {Array[]|Object} tasks
   * @return {Function}
   */
  function reflectAll(tasks) {
    var newTasks, keys;
    if (isArray(tasks)) {
      newTasks = Array(tasks.length);
      arrayEachSync(tasks, iterate);
    } else if (tasks && typeof tasks === obj) {
      keys = nativeKeys(tasks);
      newTasks = {};
      baseEachSync(tasks, iterate, keys);
    }
    return newTasks;

    function iterate(func, key) {
      newTasks[key] = reflect(func);
    }
  }

  /**
   * @memberof async
   * @namespace createLogger
   */
  function createLogger(name) {
    return function(fn) {
      var args = slice(arguments, 1);
      args.push(done);
      fn.apply(null, args);
    };

    function done(err) {
      if (typeof console === obj) {
        if (err) {
          if (console.error) {
            console.error(err);
          }
          return;
        }
        if (console[name]) {
          var args = slice(arguments, 1);
          arrayEachSync(args, function(arg) {
            console[name](arg);
          });
        }
      }
    }
  }

  /**
   * @memberof async
   * @namespace safe
   */
  function safe() {
    createImmediate();
    return exports;
  }

  /**
   * @memberof async
   * @namespace fast
   */
  function fast() {
    createImmediate(false);
    return exports;
  }
});