构造函数

es5

 function P1(x,y){
   this.x = x;
   this.y= y;
 }
 P1.prototype.run = function(){
   console.log('run跑起来，x',this.x);
 }
 let p1 = new P1(99,88);
 p1.run();
 P1.xx = '挂载到函数上';
 console.log('p1.xx',p1.xx)

es6

 class P2{
   static xx = '挂载到类上';
   //构造函数中只做一件事：初始化数据
   constructor(x,y){
    this.x = x;
    this.y= y;
   }
   run(){
    console.log('run跑起来，x',this.x);
   }
 }
 let p2 = new P2(99,88);
 p2.run();
 console.log('p2.xx',p2.xx);

继承

 class A {
   constructor(a){
     console.log('父类构造函数执行了');
     this.a = a;
   }
 }
 //继承只能单继承
 class B extends A {
   constructor(c,d){
     //需要执行父类构造函数：让我们具备a属性
     super(d);
     this.b = c;
     console.log('b中的this',this);
   }
 }
 //如果出现同名属性：优先子类
 let b1 = new B('c','d');
 console.log(b1);

 let a1 = new A('f');
 console.log(a1);