es6.array.from.js

'use strict';
var ctx         = require('./_ctx')
  , $export     = require('./_export')
  , toObject    = require('./_to-object')
  , call        = require('./_iter-call')
  , isArrayIter = require('./_is-array-iter')
  , toLength    = require('./_to-length')
  , getIterFn   = require('./core.get-iterator-method');
$export($export.S + $export.F * !require('./_iter-detect')(function(iter){ Array.from(iter); }), 'Array', {
  // 22.1.2.1 Array.from(arrayLike, mapfn = undefined, thisArg = undefined)
  from: function from(arrayLike/*, mapfn = undefined, thisArg = undefined*/){
    var O       = toObject(arrayLike)
      , C       = typeof this == 'function' ? this : Array
      , aLen    = arguments.length
      , mapfn   = aLen > 1 ? arguments[1] : undefined
      , mapping = mapfn !== undefined
      , index   = 0
      , iterFn  = getIterFn(O)
      , length, result, step, iterator;
    if(mapping)mapfn = ctx(mapfn, aLen > 2 ? arguments[2] : undefined, 2);
    // if object isn't iterable or it's array with default iterator - use simple case
    if(iterFn != undefined && !(C == Array && isArrayIter(iterFn))){
      for(iterator = iterFn.call(O), result = new C; !(step = iterator.next()).done; index++){
        result[index] = mapping ? call(iterator, mapfn, [step.value, index], true) : step.value;
      }
    } else {
      length = toLength(O.length);
      for(result = new C(length); length > index; index++){
        result[index] = mapping ? mapfn(O[index], index) : O[index];
      }
    }
    result.length = index;
    return result;
  }
});