在es6之前，我们经常遇到需要手动改变this指向的问题，call? apply?bind?,三者有啥区别，搞懂原理，也就清晰了。

call

Function.prototype.my_call = function(obj, ...args){
    obj = obj || window;
    const fn = Symbol();// Symbol防止重名key
    obj[fn] = this;
    return obj[fn](...args);// 执行，返回执行值
}

apply

Function.prototype.my_apply = function(obj, args){
    obj = obj || window;
    const fn = Symbol();// Symbol防止重名key
    obj[fn] = this;
    return obj[fn](...args);// 执行，返回执行值
}

bind

Function.prototype.my_bind = function(obj, ...args){
    obj = obj || window;
    const fn = Symbol();// Symbol防止重名key
    obj[fn] = this;
    const _this = this;//保存this
    const res = function(...args1){
        if(this instanceof _this){
            this[fn] = _this;
            this[fn](...[...args, ...args1]);
            delete this[fn];
        }else{
            this[fn](...[...args, ...args1]);
            delete this[fn];
        }
    }
    res.prototype = Object.create(this.prototype);
    return res;
}