SOURCE

const PENDDING = "pedding";
const RESOLVED = "resolved";
const REJECTED = "rejected";
function myPromise(fn) {
    const that = this;
    that.status = PENDDING;
    that.value = null;
    that.reason = null;
    that.resolvedCallBacks = [];
    that.rejectedCallBacks = [];

    function resolve(value) {
        that.status = RESOLVED;
        that.value = value;
        that.resolvedCallBacks.map(cb=>cb(that.value))
    }
    function reject(reason){
        that.status = REJECTED;
        that.reason = reason;
        that.rejectedCallBacks.map(cb=>cb(that.reason))
    }
    try{
        fn(resolve,reject)
    }catch(e){
        reject(e)
    }
}

myPromise.prototype.then = function(onFullFilled,onRejected){
    const that = this;
    if(that.status===PENDDING){
        that.resolvedCallBacks.push(onFullFilled);
        that.rejectedCallBacks.push(onRejected)
    }
    if(that.status===RESOLVED){
        onFullFilled(that.value)
    }
    if(that.status===REJECTED){
        onRejected(that.reason)
    }
    return that
}

const p = new myPromise((resolve,reject)=>{
    setTimeout(()=>{
        resolve('成功')
        // reject("失败")
    },3000)
})
p.then(res=>{
    console.log(res)
},err=>{
    console.log(err)
})