原型链

    function AA() {
    }

    function BB() {
    }

    var x = BB.prototype
    console.log(x === BB.prototype);  //true

    BB.prototype = new AA()
    // new操作符创建、继承、返回一个新对象（假设为o，存在o.__proto__=AA.prototype）
    console.log(x === BB.prototype);  //false

     function AA() {
    }

    function BB() {
    }

    var x = BB.prototype
    console.log(x.constructor);

    BB.prototype = new AA()
    console.log(BB.constructor);
    console.log(BB.prototype.__proto__ === AA.prototype);
    console.log(x.constructor);

   function AA() {
    }
    var o = {}
    AA.prototype = o

    console.log(AA.prototype.hasOwnProperty('constructor')); //false
    console.log(AA.prototype.__proto__.hasOwnProperty('constructor')); //true; constructor:Object

    console.log(AA.constructor === Function);//Function   ???  
    console.log(AA.constructor);  //Function   ???  
    //为什么不是Object???

https://github.com/mqyqingfeng/Blog/issues/2