关于this指向

前言

我是歌谣最好的种树是十年前其次是现在今天继续给大家带来的是this的讲解

环境配置

npm init -y

yarn add vite -D

修改page.json配置端口

{
  "name": "demo1",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "dev": "vite --port 3002"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "vite": "^4.4.9"
  }
}

基本案例

console.log(this==window)
var a=1;
var b=function(){
    return 'function'
}
console.log(a)
console.log(b)

运行结果

类与函数

class Test1{
    constructor(){




    }
  say(){




  }
  static do(){
    
  }
}
const test=new test1()












const Test=(function(){
    function Test2(){




    }
    Test2.prototype.say=function(){




    }
    Test.do=function(){




    }
    window.Test=Test
})




const test2=new Test2()

类组件案例

class Test{
    constructor(){
        this.test=function(){
            console.log("none-static",this)
        }
    }
    test(){
        console.log("static"+this)
    }
}
const test=new Test()
test.test()
const TestA=Object.create(null)
console.log(TestA)
const TestB={}
console.log(TestB)

运行结果

继承

class Father{
    constructor(){
        this.age=18
    }
    swim(){
        console.log("Go Swimming")
    }
}
class Son extends Father{
    constructor(){
         super()
        this.hobby="playing"
    }
    study(){
        console.log(this)
        this.swim()
    }
}
const son=new Son()
son.study()

运行结果

call，apply，bind

var obj={
    a:1
}
var obj2={
    a:100
}
var a=2;
function test(b,c){
    console.log(this.a,b,c)
}
test()
test.call(obj)
test.apply(obj)
test.call(obj,3,4)
test.apply(obj,[3,4])




var test1=test.bind(obj,3,4)
test1();




var test2=test.bind(obj2,3,4)
test2();

运行结果

严格模式

'use strict'
const test=()=>{
    console.log(this)
}
function test1(){
    console.log(this)
}
const test2=function(){
    console.log(this)
}
test()
test1()
test2()

运行结果

案例

var obj={
    a:1
}
var a=2
const test=()=>{
    console.log(this.a)
}
test()
test.call(obj)
test.apply(obj)
var test1=test.bind(obj);
test1()

运行结果

箭头函数和普通函数

var obj={}
var obj1={}
var obj2={}
obj.test = () => {
    console.log(obj)
    console.log(this)
}
obj.test()
obj1.test=function(){
    var t=()=>{
        console.log(this)
    }
    t()
}
obj1.test()
obj2.test=function(){
    console.log("test",this)
    var t1=function(){
        console.log("t1",this)
        var t2=()=>{
            console.log("t2",this)
        }
        t2()
    }
    t1()
}
obj2.test()

运行结果

案例

var obj={
    a:1,
    b:2,
    c:{
        d:4,
        test3:function(){
       
            console.log(this.d,"d is")
        }
    },
    test:function(){
        console.log(this.a,"a is")
    },
    test2:test2
}
function test2(){
    console.log(this.b,"a is")
}
obj.test()
obj.test2()
obj.c.test3()

运行结果

addEventListener案例

// var OBtn=document.getElementById("J_BUTTON")
// OBtn.onclick=function(){
//     console.log(this)
// }
// OBtn.addEventListener("click",function(){
//     console.log(this)
// },false)








; (function (doc) {
    var OBtn = document.getElementById("J_BUTTON")
    function Plus(a, b) {
        this.a = a;
        this.b = b
        this.init()
    }
    Plus.prototype.init = function () {
        this.bindEvent()
    }
    Plus.prototype.bindEvent = function () {
        OBtn.addEventListener("click",this.handleBtnClick.bind(this),false)
    }
    Plus.prototype.handleBtnClick = function () {
        console.log(this.a,this.b)
        console.log(this)
    }
    window.Plus=Plus
})(document)




new Plus(3,4)

运行结果

father和son

class Father{
    constructor(){
        this.eat=this.eat.bind(this)
    }
    get fruit(){
        return 'apple'
    }
    eat(){
        console.log('i an eat'+this.fruit)
    }
}




class Son{
    get fruit(){
        return 'orange'
    }
}
const father=new Father();
const son=new Son();
father.eat()
son.eat=father.eat
son.eat()

运行结果