JavaScript 继承实现方式

原型链继承

• 将子类的原型对象指向父类的实例
• 优点：继承了父类的模板，又继承了父类的原型对象
• 缺点：
  • 无法实现多继承(因为已经指定了原型对象了)
  • 父类的所有 引用属性（info）会被所有子类共享，更改一个子类的引用属性，其他子类也会受影响
  • 创建子类时，无法向父类构造函数传参数

function Parent() {
  this.info = {
    name: 'Parent',
    age: 18,
  }
}

Parent.prototype.getInfo = function () {
  console.log(this.info)
}

function Child() {}

// 将子类的原型对象指向父类的实例
Child.prototype = new Parent()

let child = new Child()
child.info.gender = 'M'
child.getInfo() // { name: 'Parent', age: 18, gender: 'M' }

构造函数继承

• 在子类构造函数内部使用 apply 或 call 来调用父类构造函数，复制父类的实例属性给子类
• 优点：
  • 解决了原型链继承中子类实例共享父类引用对象的问题，实现多继承
  • 创建子类实例时，可以向父类传递参数
• 缺点：
  • 构造继承只能继承父类的实例属性和方法，不能继承父类原型的属性和方法（方法属性写在构造函数中，每次创建示例都会被初始化）

function Parent(name) {
  this.info = {
    name,
    hobby: ['football', 'basketball'],
  }
}

Parent.prototype.getInfo = function () {
  console.log(this.info)
}

function Child(name, age) {
  // 继承父类属性
  Parent.call(this, name)
  this.age = age
}

// 继承父类方法
Child.prototype = new Parent()

let child1 = new Child('wujie1', 19)
child1.info.hobby.push('soccer')
console.log(child1.getInfo()) // { name: 'wujie1', hobby: [ 'football', 'basketball', 'soccer' ] }
console.log(child1.age)

let child2 = new Child('wujie2', 20)
console.log(child2.getInfo()) // { name: 'wujie2', hobby: [ 'football', 'basketball' ] }
console.log(child2.age)

组合继承

• 使用原型链继承保证子类继承父类原型的属性和方法
• 使用构造继承保证子类继承父类实例的属性和方法

function Parent(name) {
  this.info = {
    name,
    hobby: ['football', 'basketball'],
  }
}

Parent.prototype.getInfo = function () {
  console.log(this.info)
}

function Child(name, age) {
  // 继承父类属性
  Parent.call(this, name)
  this.age = age
}

// 继承父类方法
Child.prototype = new Parent()

let child1 = new Child('wujie1', 19)
child1.info.hobby.push('soccer')
console.log(child1.getInfo()) // { name: 'wujie1', hobby: [ 'football', 'basketball', 'soccer' ] }
console.log(child1.age)

let child2 = new Child('wujie2', 20)
console.log(child2.getInfo()) // { name: 'wujie2', hobby: [ 'football', 'basketball' ] }
console.log(child2.age)

原型式继承

• 通过拷贝对象引用方式实现，但可能导致对象被修改

let parent = {
  name: 'parent',
  hobby: ['football', 'basketball'],
}

let child = Object.create(parent)
child.name = 'child'
child.hobby.push('soccer')

console.log(child.name) // child
console.log(child.hobby) // [ 'football', 'basketball', 'soccer' ]

寄生式继承

• 通过获取对象的浅拷贝，再对浅拷贝方法增强（添加方法），也就是在原型式寄生的基础上再添加方法

let parent = {
  name: 'parent',
  hobby: ['football', 'basketball'],
}

function clone(original) {
  let clone = Object.create(original)
  clone.getHobby = function () {
    return this.hobby
  }
  return clone
}

let child = clone(parent)
child.name = 'child'
child.hobby.push('soccer')

console.log(child.name) // child
console.log(child.hobby) // [ 'football', 'basketball', 'soccer' ]
console.log(child.getHobby()) // [ 'football', 'basketball', 'soccer' ]

寄生组合式继承

• 将组合继承，寄生式继承组合起来实现的继承，是所有继承方式的最优解
• 优点：解决了组合继承父类会被调用两次和属性在不同层级会重复的问题

function Parent() {
  this.name = 'parent'
  this.hobby = ['football', 'basketball']
}

Parent.prototype.getHobby = function () {
  return this.hobby
}

function Child() {
  Parent.call(this)
  this.friend = 'child friends'
}

function clone(parent, child) {
  child.prototype = Object.create(parent.prototype)
  child.prototype.constructor = child
}

clone(Parent, Child)

Child.prototype.getFriend = function () {
  return this.friend
}

let child = new Child()
console.log(child.getHobby()) // [ 'football', 'basketball' ]
console.log(child.getFriend()) // child friend

class 继承

• 通过 extends、super 实现

class Parent {
  constructor(name) {
    this.name = name
  }
  getName() {
    console.log(this.name)
  }
}
class Child extends Parent {
  constructor(name) {
    super(name)
    this.age = 18
  }
}