Handwriting apply ()

apply () method calls a function of a given value of this parameter as an array (or array-like objects) is provided with, as well.
Syntax: func.apply (thisArg, [argsArray])
apply (), and call () is similar except that the call () receiving a list of parameters, and apply () receives a parameter array, we achieve a simple change in the call () of We can look into the reference form

Function.prototype.myApply = function(thisArg, args) {
    if(typeof this !== 'function') {
        throw new TypeError('error')
    }
    const fn = Symbol('fn')        // 声明一个独有的Symbol属性, 防止fn覆盖已有属性
    thisArg = thisArg || window    // 若没有传入this, 默认绑定window对象
    thisArg[fn] = this              // this指向调用call的对象,即我们要改变this指向的函数
    const result = thisArg[fn](...args)  // 执行当前函数
    delete thisArg[fn]              // 删除我们声明的fn属性
    return result                  // 返回函数执行结果
}

//测试
foo.myApply(obj, [])     // 输出'写代码像蔡徐抻'

This article reprinted from: https://juejin.im/post/5e8b261ae51d4546c0382ab4