wiki/client/polyfills/array-from.js

// 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
    }
  }())
}