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// Production steps of ECMA-262, Edition 6, 22.1.2.1
if (!Array.from) { Array.from = (function () { var toStr = Object.prototype.toString var isCallable = function (fn) { return typeof fn === 'function' || toStr.call(fn) === '[object Function]' } var toInteger = function (value) { var number = Number(value) if (isNaN(number)) { return 0 } if (number === 0 || !isFinite(number)) { return number } return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number)) } var maxSafeInteger = Math.pow(2, 53) - 1 var toLength = function (value) { var len = toInteger(value) return Math.min(Math.max(len, 0), maxSafeInteger) }
// The length property of the from method is 1.
return function from (arrayLike/*, mapFn, thisArg */) { // 1. Let C be the this value.
var C = this
// 2. Let items be ToObject(arrayLike).
var items = Object(arrayLike)
// 3. ReturnIfAbrupt(items).
if (arrayLike == null) { throw new TypeError('Array.from requires an array-like object - not null or undefined') }
// 4. If mapfn is undefined, then let mapping be false.
var mapFn = arguments.length > 1 ? arguments[1] : void undefined var T if (typeof mapFn !== 'undefined') { // 5. else
// 5. a If IsCallable(mapfn) is false, throw a TypeError exception.
if (!isCallable(mapFn)) { throw new TypeError('Array.from: when provided, the second argument must be a function') }
// 5. b. If thisArg was supplied, let T be thisArg; else let T be undefined.
if (arguments.length > 2) { T = arguments[2] } }
// 10. Let lenValue be Get(items, "length").
// 11. Let len be ToLength(lenValue).
var len = toLength(items.length)
// 13. If IsConstructor(C) is true, then
// 13. a. Let A be the result of calling the [[Construct]] internal method
// of C with an argument list containing the single item len.
// 14. a. Else, Let A be ArrayCreate(len).
var A = isCallable(C) ? Object(new C(len)) : new Array(len)
// 16. Let k be 0.
var k = 0 // 17. Repeat, while k < len… (also steps a - h)
var kValue while (k < len) { kValue = items[k] if (mapFn) { A[k] = typeof T === 'undefined' ? mapFn(kValue, k) : mapFn.call(T, kValue, k) } else { A[k] = kValue } k += 1 } // 18. Let putStatus be Put(A, "length", len, true).
A.length = len // 20. Return A.
return A } }()) }
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