实现自己的call apply bind

1.call

Function.prototype.myCall = function(context = window,...args) {
   let fn = this,
       res = null;
    context.$fn = fn;
    res = context.$fn(...args);
    delete context.$fn;
    return res;
}

注意：一个call是让左边函数执行（this是传递的参数），多个call是让最后传参的函数执行（this是window/undefined）

function fn1(){ console.log(1) }
function fn2(){ console.log(2) }
fn1.call(fn2)
fn1.call.call(fn2)
Function.prototype.call(fn1)
Function.prototype.call.call(fn1)
//输出结果：
//1  2  无输出  1

2.apply

Function.prototype.myApply = function(context = window,args) {
     let fn = this,
       res = null;
   context.$fn = fn;
   res = context.$fn(...args);
   delete context.$fn;
   return res;
}

3.bind

Function.prototype.myBind = function(context = window,...outerArg) {
    let fn = this,
      fnNOP = function(){};
  context.$fn = fn;
  let fnBound = function(...innerArgs) {
    res = fn.apply(this instanceof fn ? this : context,outerArgs.concat(innerArgs))
  }
  fnNOP.prototype = fn.prototype;
  fnBound.prototype = new fnNOP();
  return fnBound;
}