from_constructor.js

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/* Any copyright is dedicated to the Public Domain.

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

// Array.from can be applied to any constructor.

// For example, the Date builtin constructor.

var d = Array.from.call(Date, ["A", "B"]);

assertEq(Array.isArray(d), false);

assertEq(Object.prototype.toString.call(d), "[object Date]");

assertEq(Object.getPrototypeOf(d), Date.prototype);

assertEq(d.length, 2);

assertEq(d[0], "A");

assertEq(d[1], "B");

// Or Object.

var obj = Array.from.call(Object, []);

assertEq(Array.isArray(obj), false);

assertEq(Object.getPrototypeOf(obj), Object.prototype);

assertEq(Object.getOwnPropertyNames(obj).join(","), "length");

assertEq(obj.length, 0);

// Or any JS function.

function C(arg) {

    this.args = arguments;

var c = Array.from.call(C, {length: 1, 0: "zero"});

assertEq(c instanceof C, true);

assertEq(c.args.length, 1);

assertEq(c.args[0], 1);

assertEq(c.length, 1);

assertEq(c[0], "zero");

// If the 'this' value passed to Array.from is not a constructor,

// a plain Array is created.

var arr = [3, 4, 5];

var nonconstructors = [

    {}, Math, Object.getPrototypeOf, undefined, 17,

    () => ({})  // arrow functions are not constructors

];

for (var v of nonconstructors) {

    obj = Array.from.call(v, arr);

    assertEq(Array.isArray(obj), true);

    assertDeepEq(obj, arr);

// Array.from does not get confused if global.Array is replaced with another

// constructor.

function NotArray() {

var RealArray = Array;

NotArray.from = Array.from;

Array = NotArray;

assertEq(RealArray.from([1]) instanceof RealArray, true);

assertEq(NotArray.from([1]) instanceof NotArray, true);

if (typeof reportCompare === 'function')

    reportCompare(0, 0);