手写简易Promise

    // 手写promise
    function MyPromise(fn){
        let _this = this
        _this.status = "pending"
        _this.resolve_val = undefined
        _this.reject_val = undefined
        _this.resolveCb = []
        _this.rejectCb = []
        
        try{
            fn(resolve,reject)
        }catch(e){
            reject(e)
        }
        
        function resolve(value){
            if(_this.status === "pending"){
                _this.status = "resolve"
                _this.resolve_val = value
                _this.resolveCb.map(cb=>cb(value))
            }
        }
        
        function reject(value){
            if(_this.status === "pending"){
                _this.status = "reject"
                _this.reject_val = value
                _this.rejectCb.map(cb=>cb(value))
            }
        }
         
        
        MyPromise.prototype.then = function(success,fail){
            if(_this.status === "pending"){
                _this.resolveCb.push(success)
                _this.rejectCb.push(fail)
            }
            
            if(_this.status === "resolve"){
                success(_this.resolve_val)
            }
            
            if(_this.status === "reject"){
                fail(_this.resolve_val)
            }
            return this
        }
        
    }

    let p = new MyPromise((resolve,reject)=>{
        setTimeout(()=>{
            reject("你好")
        },2000)
    })
    p.then((e)=>{
        console.log(e)
    },(err)=>{
        console.log(err)
    })