js中 new运算符解密

new 为一元运算符

作用：后面只能跟函数，用来调用函数

使用new调用函数与直接调用函数的区别：

         function fn1(){
             console.log(this);
         }
         //this指向的变化
         var ret1 = fn1();// Window
         var ret2 = new fn1();//fn1{}
         //返回值的变化没有return
         console.log("ret1:"+ret1);//ret1:undefined
         console.log("ret2:"+ret2);//ret2:[object Object]
         //返回值的变化有return 返回对象
         function fn2(){
             return ["summer","autumn"];
         }
         var ret3 = fn2();
         var ret4 = new fn2();
         console.log("ret3:"+ret3);//et3:summer,autumn
         console.log("ret4:"+ret4);//ret4:summer,autumn
         //返回值的变化有return 返回非对象
         function fn3(){
             return "我是字符串";
         }
         var ret5 = fn3();
         var ret6 = new fn3();
         console.log("ret5:"+ret5);//ret5:我是字符串
         console.log("ret6:"+ret6);//ret6:[object Object]

使用new调用函数可以改变 this 的指向，返回值始终是一个对象。