JSTests/test262/test/built-ins/Array/prototype/concat/is-concat-spreadable-proxy.js

// Copyright (C) 2016 the V8 project authors. All rights reserved.
// This code is governed by the BSD license found in the LICENSE file.
/*---
esid: sec-isconcatspreadable
es6id: 22.1.3.1.1
description: >
  Proxies who final targets are arrays are considered spreadable
info: |
  [...]
  5. Repeat, while items is not empty
     a. Remove the first element from items and let E be the value of the
        element.
     b. Let spreadable be ? IsConcatSpreadable(E).
     c. If spreadable is true, then
        [...]
     e. Else E is added as a single item rather than spread,
        [...]

  ES6 22.1.3.1.1: Runtime Semantics: IsConcatSpreadable ( O )

  1. If Type(O) is not Object, return false.
  2. Let spreadable be ? Get(O, @@isConcatSpreadable).
  3. If spreadable is not undefined, return ToBoolean(spreadable).
  4. Return ? IsArray(O).

  7.2.2 IsArray

  [...]
  3. If argument is a Proxy exotic object, then
     a. If the value of the [[ProxyHandler]] internal slot of argument is null,
        throw a TypeError exception.
     b. Let target be the value of the [[ProxyTarget]] internal slot of
        argument.
     c. Return ? IsArray(target).
features: [Proxy, Symbol.isConcatSpreadable]
---*/

var arrayProxy = new Proxy([], {});
var arrayProxyProxy = new Proxy(arrayProxy, {});
var spreadable = {};
spreadable[Symbol.isConcatSpreadable] = true;
var spreadableProxy = new Proxy(spreadable, {});

assert.sameValue([].concat(arrayProxy).length, 0, 'Proxy for an array');
assert.sameValue(
  [].concat(arrayProxyProxy).length, 0, 'Proxy for a proxy for an array'
);
assert.sameValue(
  [].concat(spreadableProxy).length,
  0,
  'Proxy for an ordinary object with a truthy @@isConcatSpreadable property'
);