Javascript new 对象的过程

首先看一下下面的代码

function User(name , age, career) {
    this.name = name
    this.age = age
    this.career = career 
}
const user = new User('ryan', 19, 'fe')
console.log(user)  // {name: "ryan", age: 19, career: "fe"}

function newUser(name , age, career) {
  return {
    name,
    age,
    career ,
  }
}
const newuser = newUser('ryan', 19, 'fe')
console.log(newuser)  // {name: "ryan", age: 19, career: "fe"}

想想这两种获取对象的方式有什么不同？

再看看下面的代码

function User(name , age, career) {
    this.name = name
    this.age = age
    this.career = career 
}
User.prototype.sayName = function () {
    console.log("Hello")
}
const user = new User('ryan', 19, 'fe')
console.log(user)  // {name: "ryan", age: 19, career: "fe"}
user.sayName() // Hello

function newUser(name , age, career) {
  return {
    name,
    age,
    career ,
  }
}
newUser.prototype.sayName = function () {
    console.log("Hello")
}
const newuser = newUser('ryan', 19, 'fe')
console.log(newuser)  // {name: "ryan", age: 19, career: "fe"}
newuser.sayName() // Uncaught TypeError: newuser.sayName is not a function

看到这里是不是感觉好像明白了什么

new 的过程和普通的函数返回不同，new 其实是一种继承的方式， 通过new，实例与构造函数通过原型链连接了起来出来，所以实例能访问到构造函数的属性，具体过程如下

1. 开辟一个块内存，创建一个空对象
2. 执行构造函数，对这个空对象进行构造
3. 给这个空对象添加proto属性

  __proto__ = User.prototype