deps/v8/src/js/array.js
function ArrayFrom(arrayLike, mapfn, receiver) {
  var items = TO_OBJECT(arrayLike);
  var mapping = !IS_UNDEFINED(mapfn);

  if (mapping) {
    if (!IS_CALLABLE(mapfn)) {
      throw MakeTypeError(kCalledNonCallable, mapfn);
    }
  }

  var iterable = GetMethod(items, iteratorSymbol);
  var k;
  var result;
  var mappedValue;
  var nextValue;

  if (!IS_UNDEFINED(iterable)) {
    result = %IsConstructor(this) ? new this() : [];
    k = 0;

    for (nextValue of
         { [iteratorSymbol]() { return GetIterator(items, iterable) } }) {
      if (mapping) {
        mappedValue = %_Call(mapfn, receiver, nextValue, k);
      } else {
        mappedValue = nextValue;
      }
      %CreateDataProperty(result, k, mappedValue);
      k++;
    }
    result.length = k;
    return result;
  } else {
    var len = TO_LENGTH(items.length);
    result = %IsConstructor(this) ? new this(len) : new GlobalArray(len);

    for (k = 0; k < len; ++k) {
      nextValue = items[k];
      if (mapping) {
        mappedValue = %_Call(mapfn, receiver, nextValue, k);
      } else {
        mappedValue = nextValue;
      }
      %CreateDataProperty(result, k, mappedValue);
    }

    result.length = k;
    return result;
  }
}
function ArrayFrom(arrayLike, mapfn, receiver) {
  var items = TO_OBJECT(arrayLike);
  var mapping = !IS_UNDEFINED(mapfn);

  if (mapping) {
    if (!IS_CALLABLE(mapfn)) {
      throw MakeTypeError(kCalledNonCallable, mapfn);
    }
  }

  var iterable = GetMethod(items, iteratorSymbol);
  var k;
  var result;
  var mappedValue;
  var nextValue;

  if (!IS_UNDEFINED(iterable)) {
    result = %IsConstructor(this) ? new this() : [];
    k = 0;

    for (nextValue of
         { [iteratorSymbol]() { return GetIterator(items, iterable) } }) {
      if (mapping) {
        mappedValue = %_Call(mapfn, receiver, nextValue, k);
      } else {
        mappedValue = nextValue;
      }
      %CreateDataProperty(result, k, mappedValue);
      k++;
    }
    result.length = k;
    return result;
  } else {
    var len = TO_LENGTH(items.length);
    result = %IsConstructor(this) ? new this(len) : new GlobalArray(len);

    for (k = 0; k < len; ++k) {
      nextValue = items[k];
      if (mapping) {
        mappedValue = %_Call(mapfn, receiver, nextValue, k);
      } else {
        mappedValue = nextValue;
      }
      %CreateDataProperty(result, k, mappedValue);
    }

    result.length = k;
    return result;
  }
}
ES6, draft 10-14-14, section 22.1.2.1