es6.reflect.construct.js

// 26.1.2 Reflect.construct(target, argumentsList [, newTarget])
var $export = require('./_export');
var create = require('./_object-create');
var aFunction = require('./_a-function');
var anObject = require('./_an-object');
var isObject = require('./_is-object');
var fails = require('./_fails');
var bind = require('./_bind');
var rConstruct = (require('./_global').Reflect || {}).construct;

// MS Edge supports only 2 arguments and argumentsList argument is optional
// FF Nightly sets third argument as `new.target`, but does not create `this` from it
var NEW_TARGET_BUG = fails(function () {
  function F() { /* empty */ }
  return !(rConstruct(function () { /* empty */ }, [], F) instanceof F);
});
var ARGS_BUG = !fails(function () {
  rConstruct(function () { /* empty */ });
});

$export($export.S + $export.F * (NEW_TARGET_BUG || ARGS_BUG), 'Reflect', {
  construct: function construct(Target, args /* , newTarget */) {
    aFunction(Target);
    anObject(args);
    var newTarget = arguments.length < 3 ? Target : aFunction(arguments[2]);
    if (ARGS_BUG && !NEW_TARGET_BUG) return rConstruct(Target, args, newTarget);
    if (Target == newTarget) {
      // w/o altered newTarget, optimization for 0-4 arguments
      switch (args.length) {
        case 0: return new Target();
        case 1: return new Target(args[0]);
        case 2: return new Target(args[0], args[1]);
        case 3: return new Target(args[0], args[1], args[2]);
        case 4: return new Target(args[0], args[1], args[2], args[3]);
      }
      // w/o altered newTarget, lot of arguments case
      var $args = [null];
      $args.push.apply($args, args);
      return new (bind.apply(Target, $args))();
    }
    // with altered newTarget, not support built-in constructors
    var proto = newTarget.prototype;
    var instance = create(isObject(proto) ? proto : Object.prototype);
    var result = Function.apply.call(Target, instance, args);
    return isObject(result) ? result : instance;
  }
});