concat-spreadable-basic.js

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/* Any copyright is dedicated to the Public Domain.

 * http://creativecommons.org/licenses/publicdomain/ */

"use strict";

const x = Object.freeze([1, 2, 3]);

let fakeArray = {

    [Symbol.isConcatSpreadable]: true,

    length: 2,

    0: "hello",

    1: "world"

assertDeepEq(x.concat(fakeArray), [1, 2, 3, "hello", "world"]);

assertDeepEq(x.concat(fakeArray, fakeArray), [1, 2, 3, "hello", "world", "hello", "world"]);

for (let truthy of [true, 3.41, "abc", Symbol(), {}]) {

    let obj = {[Symbol.isConcatSpreadable]: truthy, length: 1, 0: "hey"}

    assertDeepEq(x.concat(obj), [1, 2, 3, "hey"]);

for (let notTruthy of [null, undefined, false, 0, NaN, ""]) {

    let obj = {[Symbol.isConcatSpreadable]: notTruthy, length: 1, 0: "hey"}

    assertDeepEq(x.concat(obj), [1, 2, 3, obj]);

let array = [5, 4];

assertDeepEq(x.concat(array), [1, 2, 3, 5, 4]);

// Can make arrays non-spreadable

array[Symbol.isConcatSpreadable] = false;

assertDeepEq(x.concat(array), [1, 2, 3, [5, 4]]);

// Explicitly spreadable

array[Symbol.isConcatSpreadable] = true;

assertDeepEq(x.concat(array), [1, 2, 3, 5, 4]);

reportCompare(true, true);