core-js/internals/array-from.js

'use strict';
var bind = require('../internals/function-bind-context');
var call = require('../internals/function-call');
var toObject = require('../internals/to-object');
var callWithSafeIterationClosing = require('../internals/call-with-safe-iteration-closing');
var isArrayIteratorMethod = require('../internals/is-array-iterator-method');
var isConstructor = require('../internals/is-constructor');
var lengthOfArrayLike = require('../internals/length-of-array-like');
var createProperty = require('../internals/create-property');
var getIterator = require('../internals/get-iterator');
var getIteratorMethod = require('../internals/get-iterator-method');

var $Array = Array;

// `Array.from` method implementation
// https://tc39.es/ecma262/#sec-array.from
module.exports = function from(arrayLike /* , mapfn = undefined, thisArg = undefined */) {
  var O = toObject(arrayLike);
  var IS_CONSTRUCTOR = isConstructor(this);
  var argumentsLength = arguments.length;
  var mapfn = argumentsLength > 1 ? arguments[1] : undefined;
  var mapping = mapfn !== undefined;
  if (mapping) mapfn = bind(mapfn, argumentsLength > 2 ? arguments[2] : undefined);
  var iteratorMethod = getIteratorMethod(O);
  var index = 0;
  var length, result, step, iterator, next, value;
  // if the target is not iterable or it's an array with the default iterator - use a simple case
  if (iteratorMethod && !(this === $Array && isArrayIteratorMethod(iteratorMethod))) {
    result = IS_CONSTRUCTOR ? new this() : [];
    iterator = getIterator(O, iteratorMethod);
    next = iterator.next;
    for (;!(step = call(next, iterator)).done; index++) {
      value = mapping ? callWithSafeIterationClosing(iterator, mapfn, [step.value, index], true) : step.value;
      createProperty(result, index, value);
    }
  } else {
    length = lengthOfArrayLike(O);
    result = IS_CONSTRUCTOR ? new this(length) : $Array(length);
    for (;length > index; index++) {
      value = mapping ? mapfn(O[index], index) : O[index];
      createProperty(result, index, value);
    }
  }
  result.length = index;
  return result;
};