考察构造函数

对原型链的考察

 function func(name){
        this.name= name|| "sapi";
        function say() {
            console.log("hello" + this.name)
        }
    }
    func.prototype.say = function (prefix) {
        console.log(prefix +this.name)
    }
    new func().say()    //undefinedsapi