【js】5种this绑定简析 javascriptthis

this绑定/this指向

默认绑定 - window
隐式绑定 - context
显示绑定 - call、apply、bind
new绑定 - 新对象
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

【【js】5种this绑定简析】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

参数传递
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

bind
call
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
软绑定：本次调用时为绑定，下次执行还需要再次绑定
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，同样也是软绑定
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