手撸一个promise【js】

秒速五厘米 2024-03-26 14:12 100阅读 0赞

class myPromise{
        static PENDING = "进行中";
        static RESOLVED = "完成";
        static REJECTED = "拒绝";
        constructor(func){
            //防止resolve被异步了，存储then的参数函数，等reslove完成再执行
            this.resStack = []
            this.rejStack = []
    
            this.status = myPromise.PENDING;
            this.result = null; //用来存储给then或者catch用
            //对应我们在promise时传入的函数：new Promise((resolve,reject) => {...})
            //需要的是，我们给他们绑定作用域，不然找不到status
            func(this.resolve.bind(this),this.reject.bind(this));
        }
        resolve(result){
            //需要保证resolve再函数循环执行队列的最后执行，所以再加一个计时器包裹
            setTimeout(()=>{
                        //只有在进行的时候才能执行resove，并对其改变状态
                        if(this.status === myPromise.PENDING){
                            this.status = myPromise.RESOLVED
                            this.result = result
                            //判断有无待执行的回调
                            this.resStack.forEach(callback=>{callback(result)})
                        }
            })
        }
        reject(result){
            //需要保证reject再函数循环执行队列的最后执行，所以再加一个计时器包裹
            setTimeout(()=>{
                        //只有在进行的时候才能执行reject，并对其改变状态
                        if(this.status === myPromise.PENDING){
                            this.status = myPromise.REJECTED
                            this.result = "error"
                            this.rejStack.forEach(callback=>{callback(result)})
                        }
            })
        }
        then(funcResolve,funcReject){
            //如果传入的参数不是函数的话我们需要先转化成函数
            funcReject =  typeof funcReject === 'function' ? funcReject : ()=>{}
            funcResolve =  typeof funcResolve === 'function' ? funcResolve : ()=>{}
    
            if(this.status === myPromise.PENDING){
                //保存回调到resolve中执行，让then等等resolve
                this.rejStack.push(funcReject)
                this.resStack.push(funcResolve)
            }else if(this.status === myPromise.RESOLVED){
                setTimeout(()=>funcResolve(this.result))//添加异步
            }
            else setTimeout(funcReject(this.result))
            //then返回还是promise
            return this
        }
        catch(func){
            if(this.status === myPromise.REJECTED){
                func(this.result)
            }
        }
    }
    
    let mp = new myPromise((resolve,reject)=>{
        setTimeout(()=>{resolve('111')})
        // resolve('111')
    }).then((result)=>{
        console.log(result)
    }).then((result)=>{
        console.log("ooooo")
    })