es6 promise原理是什么？

es6 promise原理是什么？
es6 promise实现可以简单写一写么？55

// 三个常量⽤于表示状态
const PENDING = 'pending'
const RESOLVED = 'resolved'
const REJECTED = 'rejected'
function MyPromise(fn) {
const that = this
this.state = PENDING
// value 变量⽤于保存 resolve 或者 reject 中传⼊的值
this.value = null
// ⽤于保存 then 中的回调，因为当执⾏完 Promise 时状态可能还是等待中，这时候应该把
that.resolvedCallbacks = []
that.rejectedCallbacks = []
function resolve(value) {
// ⾸先两个函数都得判断当前状态是否为等待中
if(that.state === PENDING) {
that.state = RESOLVED
that.value = value
// 遍历回调数组并执⾏
that.resolvedCallbacks.map(cb=>cb(that.value))
}
}
function reject(value) {
if(that.state === PENDING) {
that.state = REJECTED
that.value = value
that.rejectedCallbacks.map(cb=>cb(that.value))
}
}
// 完成以上两个函数以后，我们就该实现如何执⾏ Promise 中传⼊的函数了
try {
fn(resolve,reject)
}cach(e){
reject(e)
}
}
// 最后我们来实现较为复杂的 then 函数
MyPromise.prototype.then = function(onFulfilled,onRejected){
const that = this
// 判断两个参数是否为函数类型，因为这两个参数是可选参数
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v=>v
onRejected = typeof onRejected === 'function' ? onRejected : e=>throw e
// 当状态不是等待态时，就去执⾏相对应的函数。如果状态是等待态的话，就往回调函数中 push
if(this.state === PENDING) {
this.resolvedCallbacks.push(onFulfilled)
this.rejectedCallbacks.push(onRejected)
}
if(this.state === RESOLVED) {
onFulfilled(that.value)
}
if(this.state === REJECTED) {
onRejected(that.value)
}
}

你可以参考如下链接：

https://www.cnblogs.com/minigrasshopper/p/9141307.html
https://blog.csdn.net/grapelove01/article/details/80506327

如果对你有帮助，可以给我个采纳吗，谢谢!! 点击我这个回答右上方的【采纳】按钮

es6 简单实现讲解一下？

<script type="text/javascript">
    

    function P(fn){
        this.f = null;
        this.ferror = null;
        this.then = (f)=>{
            this.f = f;
            fn(this.a,this.b);
            return this;
        };
        this.error = (f)=>{
            this.ferror = f;
            fn(this.a,this.b);
            return this;
        }


        this.a = (data)=>{
            if(this.f!=null){
                this.f(data);
                this.f =null;
            }
        };
        this.b = (error)=>{
            if(this.ferror!=null){
                this.ferror(error);
                this.ferror = null;
            }
        };

    }

    let p = new P(function(a,b){
        //假如成功执行的方法
        setTimeout(()=>a("成功了"),1000);
        //假如失败执行的方法
        //setTimeout(()=>b("失败了"),1000);
    })

    p.then(function(data){
        console.log(data);
    }).error(function(error){
        console.log(error);
    })
</script>
//好像有点不一样。调用then和error会执行两次不行，