写一个 构造函数 Promise

实现 Promise 的简化版，满足以下需求即可

function Promise(???){
    ???
    return ???
}

var promise = new Promise(function(x,y){
    setTimeout(()=>{
        x(101)
    }, 3000)
})
promise.then((z)=>{
    console.log(z)  // 101
})

Promise 简化版本

function Promise(fn){
    var status = 'pending'
    function successNotify(){
        status = 'resolved'
        toDoThen.apply(undefined, arguments)
    }
    function failNotify(){
        status = 'rejected'
        toDoThen.apply(undefined, arguments)
    }
    function toDoThen(){
        setTimeout(()=>{ // 保证回调是异步执行的
            if(status === 'resolved'){
                for(let i =0; i< successArray.length;i ++)    {
                    successArray[i].apply(undefined, arguments)
                }
            }else if(status === 'rejected'){
                for(let i =0; i< failArray.length;i ++)    {
                    failArray[i].apply(undefined, arguments)
                }
            }
        })

    }
    var successArray = []
    var failArray = []
    fn.call(undefined, successNotify, failNotify)
    return {
        then: function(successFn, failFn){
            successArray.push(successFn)
            failArray.push(failFn)
            return undefined // 简化
        }
    }
}

时序图