src/v8/test/mjsunit/harmony/array-concat-object-proto-dict-getter.js

// Copyright 2016 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Check that @@isConcatSpreadable is checked when set on Object.prototype
// with a dictionary backing store.

// Force Object.prototype into dictionary backing store by adding many
// properties.
for (var i = 0; i < 10*1000; i++) {
  Object.prototype['generatedProperty'+i] = true;
}

var array = [1, 2, 3];
var object = {length: 1, '0': 'a'};

function testConcatDefaults() {
  assertEquals(array, [].concat(array));
  assertEquals(array, array.concat([]));
  assertEquals([1, 2, 3, 1, 2, 3], array.concat(array));
  assertEquals([object], [].concat(object));
  assertEquals([1, 2, 3, object], array.concat(object));
  assertEquals([object], Array.prototype.concat.call(object,[]));
  assertEquals([object, 1, 2, 3], Array.prototype.concat.call(object, array));
  assertEquals([object, object], Array.prototype.concat.call(object, object));
}

testConcatDefaults();

var concatSpreadable = false;
Object.defineProperty(Object.prototype, Symbol.isConcatSpreadable, {
  get() { return concatSpreadable },
  configurable: true
});

assertEquals([[], array], [].concat(array));
assertEquals([array, []], array.concat([]));
assertEquals([array, array], array.concat(array));
assertEquals([[], object], [].concat(object));
assertEquals([array, object], array.concat(object));
assertEquals([object, []], Array.prototype.concat.call(object,[]));
assertEquals([object, array], Array.prototype.concat.call(object, array));
assertEquals([object, object], Array.prototype.concat.call(object, object));

concatSpreadable = true;

assertEquals(array, [].concat(array));
assertEquals(array, array.concat([]));
assertEquals([1, 2, 3, 1, 2, 3], array.concat(array));
assertEquals(['a'], [].concat(object));
assertEquals([1, 2, 3, 'a'], array.concat(object));
assertEquals(['a'], Array.prototype.concat.call(object,[]));
assertEquals(['a', 1, 2, 3], Array.prototype.concat.call(object, array));
assertEquals(['a', 'a'], Array.prototype.concat.call(object, object));

delete Object.prototype[Symbol.isConcatSpreadable];
testConcatDefaults();