apply

function myApply(obj,args){
    /*
    1. this指向调用者
    2. 使用隐式绑定 obj.fn = this
    3. obj.fn()
    4. 调用完以后删除fn
    */
    // 确保obj是一个对象
    obj = (obj===null || obj === undefined) ? window:Object(obj)
    
    
    obj.fn = this
    obj.fn(...args)
    delete obj.fn
}

// 添加到Function原型上
Function.prototype.myApply = myApply

call

Function.prototype.myCall = function(obj,...args){
    obj = (obj===null || obj === undefined) ? window:Object(obj)
    obj.fn = this
    obj.fn(...args)
    delete obj.fn
}

bind

Function.prototype.myBind = function(obj,...args){
    // 判断是否为对象类型
    obj = obj ===null||obj ===undefined ? window:Object(obj)
    obj.fn = this
    return (...fnArgs)=>{
        obj.fn(...args,...fnArgs)
    }
}

new

function myNew(fn){
    /*
    0. 判断是否是一个函数
    1. 创建一个空对象,对象的原型指向fn的原型
    2. fn绑定this
    3. 返回对象或fn的返回值
    */
    if (typeof fn !== 'function') {
        throw 'must be a function'
    }
    var obj = Object.create(fn.prototype)
    var args = [...arguments].slice(1)
    var result = fn.apply(obj,args)
    
    if ((typeof result ==='object' && typeof result !== null) || typeof result ==='function') {
        return result
    }
    return obj
}