ja apply()用法及手写apply()

本文主要是介绍ja apply()用法及手写apply()，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1、apply()用法

改变this指向

fun.apply(obj) 把fun函数中的this指向了obj。obj为空或undefind就是window

    var obj = {a: 1}function fun () {console.log(this.a)}fun() //undefinedfun.apply(obj) //1

    var w = 'w'function fun () {console.log(this.w)}fun() //wfun.apply() //w

一般入参

    var obj = {a: 1}function fun (b, c) {console.log(this.a)console.log(b)console.log(c)}fun() //undefinedfun.apply(obj, [2, 3]) //1 2 3

2、手写apply()

    var obj = {a: 1}function fun (b, c) {console.log(this.a)console.log(b, c)}function myApply (fun, obj, arr) {obj = obj || windowobj.fun = funlet resultif (arr) {result= obj.fun(...arr)} else {result= obj.fun()}delete obj.fun //为了避免内存泄漏或者清除不再需要的函数引用return result}myApply(fun, obj, [2, 3])

如果要给函数对象添加新方法如下：

    var obj = {a: 1}function fun (b, c) {console.log(this.a)console.log(b, c)}// Function.prototype.myApply = function (obj) {//   obj = obj || window;//   let fn;//   obj[fn] = this;//   let result;//   result = obj[fn](...arguments[1]);//js把传入到这个函数的全部参数存储在一个叫做arguments的数组里//   delete obj[fn];//为了避免内存泄漏或者清除不再需要的函数引用//   return result// };//或Function.prototype.myApply = function (obj, arg) {obj = obj || window;let fn;obj[fn] = this;let result;result = obj[fn](...args);delete obj[fn];return result}fun.apply(obj, [2, 3]) //1 2 3

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