Promise实现（更新中）

水深无声 2023-06-11 14:25 31阅读 0赞

## 第一版 ##

// Promise.js
    const PENDING = 'pending';
    const FULLFILLED='fullfilled'
    const REJECTED='rejected'
    function Promise(excutor){ 
        let self = this; //缓存当前Promise实例
        self.status='pending';//设置状态
        //定义成功回调的数组
        self.onResolvedCallbacks=[];
        //定义失败回调的数组
        self.onRejectedCallbacks=[];
    
        //如果为pending，就转化为成功态
        function resolve(value){ 
            if(self.status==PENDING){ 
                self.status=FULLFILLED;
                self.value=value;
                self.onResolvedCallbacks.forEach(cb => cb(self.value));
            }
        }
    
        //如果为pending，就转化为失败态
        function reject(reason){ 
            if(self.status==PENDING){ 
                self.status=REJECTED;
                self.value=reason;
                self.onRejectedCallbacks.forEach(cb => cb(self.value));
            }
        }
    
        try{ 
            excutor(resolve,reject)
        }
        catch(e){ 
            reject(e)
        }
    }
    
    Promise.prototype.then=function(onFullied,onRejected){ 
        
        let self=this;
    
        //对应的业务是new Promise().then()，then()里面没有写任何逻辑，此时执行value=>value，或者reason=>{throw reason}
        onFullied=typeof(onFullied)=='function'?onFullied:value=>value
        onRejected=typeof(onRejected)=='function'?onRejected:reason=>{ throw reason}
    
        //以下两种是，new Promise之后，执行同步代码
        if(self.status==FULLFILLED){ 
            let x = onFullied(self.value) //对应then里面return
    
            //如果x是Promise
            
        }
        if(self.status==REJECTED){ 
            let x = onRejected(self.value) //对应then里面return
        }
    
        //最初将onResolvedCallbacks，onRejectedCallbacks填充函数,解决在new Promise里面使用异步函数resolve，此时调用then还没有
        //开始执行，需要将resolve(value)，reject(value)存储到数组里面，等到状态改变为fullfilled或者rejected时，循环数组，调用数组
        //里面每一个函数
        if(self.status==PENDING){ 
            self.onResolvedCallbacks.push(onFullied)
            self.onRejectedCallbacks.push(onRejected)
        }
    }
    module.exports=Promise

//case.js
    let myPromise = require('./Promise.js');
    let p1=new myPromise(function(resolve,reject){ 
        setTimeout(function(){ 
            let num = Math.random();
            if(num<0.5){ 
                resolve(num);
            }
            else{ 
                reject('失败');
            }
        })
    })
    p1.then(function(data){ 
        console.log(data)
    },function(err){ 
        console.error(err)
    })