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Filename : polyfill.js
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// Production steps of ECMA-262, Edition 6, 22.1.2.1
// Reference: http://www.ecma-international.org/ecma-262/6.0/#sec-array.from
module.exports = (function() {
  var isCallable = function(fn) {
    return typeof fn === '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);
  };
  var iteratorProp = function(value) {
    if(value != null) {
      if(['string','number','boolean','symbol'].indexOf(typeof value) > -1){
        return Symbol.iterator;
      } else if (
        (typeof Symbol !== 'undefined') &&
        ('iterator' in Symbol) &&
        (Symbol.iterator in value)
      ) {
        return Symbol.iterator;
      }
      // Support "@@iterator" placeholder, Gecko 27 to Gecko 35
      else if ('@@iterator' in value) {
        return '@@iterator';
      }
    }
  };
  var getMethod = function(O, P) {
    // Assert: IsPropertyKey(P) is true.
    if (O != null && P != null) {
      // Let func be GetV(O, P).
      var func = O[P];
      // ReturnIfAbrupt(func).
      // If func is either undefined or null, return undefined.
      if(func == null) {
        return void 0;
      }
      // If IsCallable(func) is false, throw a TypeError exception.
      if (!isCallable(func)) {
        throw new TypeError(func + ' is not a function');
      }
      return func;
    }
  };
  var iteratorStep = function(iterator) {
    // Let result be IteratorNext(iterator).
    // ReturnIfAbrupt(result).
    var result = iterator.next();
    // Let done be IteratorComplete(result).
    // ReturnIfAbrupt(done).
    var done = Boolean(result.done);
    // If done is true, return false.
    if(done) {
      return false;
    }
    // Return result.
    return result;
  };

  // The length property of the from method is 1.
  return function from(items /*, mapFn, thisArg */ ) {
    'use strict';

    // 1. Let C be the this value.
    var C = this;

    // 2. If mapfn is undefined, let mapping be false.
    var mapFn = arguments.length > 1 ? arguments[1] : void 0;

    var T;
    if (typeof mapFn !== 'undefined') {
      // 3. else
      //   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'
        );
      }

      //   b. If thisArg was supplied, let T be thisArg; else let T
      //      be undefined.
      if (arguments.length > 2) {
        T = arguments[2];
      }
      //   c. Let mapping be true (implied by mapFn)
    }

    var A, k;

    // 4. Let usingIterator be GetMethod(items, @@iterator).
    // 5. ReturnIfAbrupt(usingIterator).
    var usingIterator = getMethod(items, iteratorProp(items));

    // 6. If usingIterator is not undefined, then
    if (usingIterator !== void 0) {
      // a. If IsConstructor(C) is true, then
      //   i. Let A be the result of calling the [[Construct]]
      //      internal method of C with an empty argument list.
      // b. Else,
      //   i. Let A be the result of the abstract operation ArrayCreate
      //      with argument 0.
      // c. ReturnIfAbrupt(A).
      A = isCallable(C) ? Object(new C()) : [];

      // d. Let iterator be GetIterator(items, usingIterator).
      var iterator = usingIterator.call(items);

      // e. ReturnIfAbrupt(iterator).
      if (iterator == null) {
        throw new TypeError(
          'Array.from requires an array-like or iterable object'
        );
      }

      // f. Let k be 0.
      k = 0;

      // g. Repeat
      var next, nextValue;
      while (true) {
        // i. Let Pk be ToString(k).
        // ii. Let next be IteratorStep(iterator).
        // iii. ReturnIfAbrupt(next).
        next = iteratorStep(iterator);

        // iv. If next is false, then
        if (!next) {

          // 1. Let setStatus be Set(A, "length", k, true).
          // 2. ReturnIfAbrupt(setStatus).
          A.length = k;

          // 3. Return A.
          return A;
        }
        // v. Let nextValue be IteratorValue(next).
        // vi. ReturnIfAbrupt(nextValue)
        nextValue = next.value;

        // vii. If mapping is true, then
        //   1. Let mappedValue be Call(mapfn, T, «nextValue, k»).
        //   2. If mappedValue is an abrupt completion, return
        //      IteratorClose(iterator, mappedValue).
        //   3. Let mappedValue be mappedValue.[[value]].
        // viii. Else, let mappedValue be nextValue.
        // ix.  Let defineStatus be the result of
        //      CreateDataPropertyOrThrow(A, Pk, mappedValue).
        // x. [TODO] If defineStatus is an abrupt completion, return
        //    IteratorClose(iterator, defineStatus).
        if (mapFn) {
          A[k] = mapFn.call(T, nextValue, k);
        }
        else {
          A[k] = nextValue;
        }
        // xi. Increase k by 1.
        k++;
      }
      // 7. Assert: items is not an Iterable so assume it is
      //    an array-like object.
    } else {

      // 8. Let arrayLike be ToObject(items).
      var arrayLike = Object(items);

      // 9. ReturnIfAbrupt(items).
      if (items == null) {
        throw new TypeError(
          'Array.from requires an array-like object - not null or undefined'
        );
      }

      // 10. Let len be ToLength(Get(arrayLike, "length")).
      // 11. ReturnIfAbrupt(len).
      var len = toLength(arrayLike.length);

      // 12. If IsConstructor(C) is true, then
      //     a. Let A be Construct(C, «len»).
      // 13. Else
      //     a. Let A be ArrayCreate(len).
      // 14. ReturnIfAbrupt(A).
      A = isCallable(C) ? Object(new C(len)) : new Array(len);

      // 15. Let k be 0.
      k = 0;
      // 16. Repeat, while k < len… (also steps a - h)
      var kValue;
      while (k < len) {
        kValue = arrayLike[k];
        if (mapFn) {
          A[k] = mapFn.call(T, kValue, k);
        }
        else {
          A[k] = kValue;
        }
        k++;
      }
      // 17. Let setStatus be Set(A, "length", len, true).
      // 18. ReturnIfAbrupt(setStatus).
      A.length = len;
      // 19. Return A.
    }
    return A;
  };
})();