js 手写call apply

比较麻烦的是this指向的问题，但是可以通过"对象的函数调用指向对象自身"来处理
如果没有传入context， 那么this默认指向window

    Function.prototype._call = function (context) {
        const type = typeof this
        if (type !== "function") throw "type error: " + type
        const args = [...arguments].slice(1)
        context = context || window
        context.fn = this
        const res = context.fn(...args)
        delete context.fn
        return res
    }

    Function.prototype._apply = function (context, args) {
        const type = typeof this
        if (type !== "function" || !Array.isArray(args)) throw "type error"
        context = context || window
        context.fn = this
        const res = context.fn(...args)
        delete context.fn
        return res
    }

    const fn = function () {
        console.log(this, [...arguments]);
    }
    fn._call({ a: 1 },1,2,3,4)  // {a: 1}
    fn._call(null,1,2,3) // window 
    fn._call() // window 
    fn._apply({a:1},[12,23])  // {a: 1}