src/v8/test/mjsunit/array-shift.js

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// Check that shifting array of holes keeps it as array of holes
(function() {
  var array = new Array(10);
  array.shift();
  assertFalse(0 in array);
})();

// Now check the case with array of holes and some elements on prototype.
(function() {
  var len = 9;
  var array = new Array(len);
  Array.prototype[3] = "@3";
  Array.prototype[7] = "@7";

  assertEquals(len, array.length);
  for (var i = 0; i < array.length; i++) {
    assertEquals(array[i], Array.prototype[i]);
  }

  array.shift();

  assertEquals(len - 1, array.length);
  // Note that shift copies values from prototype into the array.
  assertEquals(array[2], Array.prototype[3]);
  assertTrue(array.hasOwnProperty(2));

  assertEquals(array[6], Array.prototype[7]);
  assertTrue(array.hasOwnProperty(6));

  // ... but keeps the rest as holes:
  Array.prototype[5] = "@5";
  assertEquals(array[5], Array.prototype[5]);
  assertFalse(array.hasOwnProperty(5));

  assertEquals(array[3], Array.prototype[3]);
  assertFalse(array.hasOwnProperty(3));

  assertEquals(array[7], Array.prototype[7]);
  assertFalse(array.hasOwnProperty(7));

  assertTrue(delete Array.prototype[3]);
  assertTrue(delete Array.prototype[5]);
  assertTrue(delete Array.prototype[7]);
})();

// Now check the case with array of holes and some elements on prototype
// which is an array itself.
(function() {
  var len = 9;
  var array = new Array(len);
  var array_proto = new Array();
  array_proto[3] = "@3";
  array_proto[7] = "@7";
  array.__proto__ = array_proto;

  assertEquals(len, array.length);
  for (var i = 0; i < array.length; i++) {
    assertEquals(array[i], array_proto[i]);
  }

  array.shift();

  assertEquals(len - 1, array.length);
  // Note that shift copies values from prototype into the array.
  assertEquals(array[2], array_proto[3]);
  assertTrue(array.hasOwnProperty(2));

  assertEquals(array[6], array_proto[7]);
  assertTrue(array.hasOwnProperty(6));

  // ... but keeps the rest as holes:
  array_proto[5] = "@5";
  assertEquals(array[5], array_proto[5]);
  assertFalse(array.hasOwnProperty(5));

  assertEquals(array[3], array_proto[3]);
  assertFalse(array.hasOwnProperty(3));

  assertEquals(array[7], array_proto[7]);
  assertFalse(array.hasOwnProperty(7));
})();

// Check that non-enumerable elements are treated appropriately
(function() {
  var array = [1, 2, 3];
  Object.defineProperty(array, '1', {enumerable: false});
  assertEquals(1, array.shift());
  assertEquals([2, 3], array);

  array = [1,,3];
  array.__proto__[1] = 2;
  Object.defineProperty(array.__proto__, '1', {enumerable: false});
  assertEquals(1, array.shift());
  assertEquals([2, 3], array);
})();