new 功能模拟

function new2(){
  var obj = {};
  var Constructor = [].shift.call(arguments);//拿到待传入的构造函数
  obj._proto_ = Constructor.prototype;
  var result = Constructor.apply(obj,arguments);
  return typeof result === "object" ? result : obj ;
}

//演示效果
function a(){
    this.x = '111'
};
var b = new2(a);
b.x  //"111"