【js】5种this绑定简析

怼怼

this绑定/this指向

  1. 默认绑定 - window
  2. 隐式绑定 - context
  3. 显示绑定 - call、apply、bind
  4. new绑定 - 新对象
  5. 箭头函数绑定 - 上层作用域

1.默认绑定

function fn1() {
    let a = 1;
    let fn2 = function () {
        console.log(this);
        console.log(this.a);
    }
    fn2();
    return fn2
}
var a = 2;
fn1() //window 2
fn1()() //window 2 window 2

无论函数在哪声明或调用,调用时函数前无对象,this最终都指向window

2.隐式绑定

function fn() {
    console.log(this.name);
}
let obj1 = {
    name: "boj1",
    fn
}
let obj2 = {
    name: "obj2",
    obj1
}
obj2.obj1.fn() //obj1

函数调用前有对象时,this就近指向上下文对象

若未寻找到属性返回undefined,不会在obj2上找

隐式丢失

  1. 参数传递
  2. 变量赋值

参数传递

let name = "window"
function fn() {
    console.log(this.name);
}
let obj1 = {
    name: "obj1",
    fn
}
let obj2 = {
    name: "obj2",
    obj1
}
function func(param) {
    param()
}
func(obj2.obj1.fn) //window
fn作为参数在func中调用,this指向丢失,默认指向window

变量赋值

let name = "window"
function fn() {
    console.log(this.name);
}
let obj1 = {
    name: "obj1",
    fn
}
let obj2 = {
    name: "obj2",
    obj1
}
let func = obj2.obj1.fn
func();//window
将fn赋给func,func的调用this默认指向window

3.显示绑定

  1. bind
  2. call
  3. apply

bind

let name = "window"
function fn() {
    console.log(this.name);
}
let obj1 = {
    name: "obj1",
    fn
}
let obj2 = {
    name: "obj2",
    obj1
}
function func1(param) {
    param()
}
func1(obj1.fn.bind(obj2)) //obj2
let func2 = obj1.fn.bind(obj2)
func2() //obj2
绑定:fn.bind(obj) 将fn绑定在obj上并返回一个函数,不管是参数传递还是变量赋值都不会出现绑定丢失的情况
参数:bind第一个参数为绑定的对象,后面的可以追加参数作为函数的实参,传递一个数组时不会自动解构
优先级:显示绑定>隐式绑定>默认绑定
硬绑定:绑定后不可使用bind、call、apply再重新绑定

call

let name = "window"
function fn() {
    let name = this.name
    return function fn1() {
        console.log(name, this.name);
    }
}
let obj1 = {
    name: "obj1",
    fn
}
let obj2 = {
    name: "obj2",
    obj1
}
function func1(param) {
    param()
}
func1(obj1.fn.call(obj2)) //obj2 window
let func2 = obj1.fn.call(obj2)
func2() //obj2 window
func2.call(obj1) //obj2 obj1
绑定:call绑定不同与bind的返回一个绑定的函数,而是立即执行这个函数
参数:与bind相同,this指向第一个参数,后续参数作为实参,数组不自动解构
闭包:在绑定时只绑定最外层作用域,父函数返回的函数在调用时为默认绑定,当对其绑定时,其this会被绑定,不影响外层作用域

apply

let arr = ["param1", "param2"]
let name = "window"
function fn(...arr) {
    console.log(this.name, arr[0], arr[1]);
}
let obj1 = {
    name: "obj1",
    fn
}
let obj2 = {
    name: "obj2",
}
obj1.fn.apply(obj2, arr)//obj2 param1 param2
绑定:apply与call相同,会立即执行被绑定的函数
参数:第一个参数也是绑定的对象,后面与call不同,apply传递的数组会解构赋给函数的的形参

4.new绑定

function fn(name) {
    this.name = name
}
let obj = new fn("obj")
console.log(obj.name); //obj
console.log(new fn("new"));// {name:new}
用new去调用函数,会将该函数作为一个构造函数,把该函数的this指向一个新创建的对象,返回该对象

5.箭头函数

可以与call中的例子对比

let name = "window"
function fn() {
    let name = this.name
    return () => {
        console.log(name, this.name);
    }
}
let obj1 = {
    name: "obj1",
}
let obj2 = {
    name: "obj2",
}
let func2 = fn.call(obj2)
func2() // obj2 obj2
func2.call(obj1) // obj2 obj2
let func1 = fn.call(obj1)
func1() //obj1 obj1
箭头函数没有自己的this,它会使用上一层作用域的this
对箭头函数的绑定是没有效果的,修改指向需要修改上层作用域的this
阅读 185
37 声望
1 粉丝
0 条评论
你知道吗?

37 声望
1 粉丝
宣传栏