首先看一下下面的代碼

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"}

想想這兩種獲取對象的方式有什么不同？

再看看下面的代碼

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

看到這里是不是感覺好像明白了什么

new 的過程和普通的函數(shù)返回不同，new 其實是一種繼承的方式， 通過new，實例與構(gòu)造函數(shù)通過原型鏈連接了起來出來，所以實例能訪問到構(gòu)造函數(shù)的屬性，具體過程如下

1. 開辟一個塊內(nèi)存，創(chuàng)建一個空對象
2. 執(zhí)行構(gòu)造函數(shù)，對這個空對象進行構(gòu)造
3. 給這個空對象添加proto屬性

  __proto__ = User.prototype