ES6 原理实现 #

function resolvePromise(promise2,x,resolve,reject){
    //判断x是不是promise
    //规范中规定：我们允许别人乱写，这个代码可以实现我们的promise和别人的promise 进行交互
    if(promise2 === x){//不能自己等待自己完成
        return reject(new TypeError('循环引用'));
    };
    // x是除了null以外的对象或者函数
    if(x !=null && (typeof x === 'object' || typeof x === 'function')){
        let called;//防止成功后调用失败
        try{//防止取then是出现异常  object.defineProperty
            let then = x.then;//取x的then方法 {then:{}}
            if(typeof then === 'function'){//如果then是函数就认为他是promise
                //call第一个参数是this，后面的是成功的回调和失败的回调
                then.call(x,y => {//如果Y是promise就继续递归promise
                    if(called) return;
                    called = true;
                    resolvePromise(promise2,y,resolve,reject)
                },r => { //只要失败了就失败了
                    if(called) return;
                    called = true;
                    reject(r);  
                });
            }else{//then是一个普通对象，就直接成功即可
                resolve(x);
            }
        }catch (e){
            if(called) return;
            called = true;
            reject(e)
        }
    }else{//x = 123 x就是一个普通值 作为下个then成功的参数
        resolve(x)
    }

}

class Promise {
    constructor (executor){
        //默认状态是等待状态
        this.status = 'panding';
        this.value = undefined;
        this.reason = undefined;
        //存放成功的回调
        this.onResolvedCallbacks = [];
        //存放失败的回调
        this.onRejectedCallbacks = [];
        let resolve = (data) => {//this指的是实例
            if(this.status === 'pending'){
                this.value = data;
                this.status = "resolved";
                this.onResolvedCallbacks.forEach(fn => fn());
            }
 
        }
        let reject = (reason) => {
            if(this.status === 'pending'){
                this.reason = reason;
                this.status = 'rejected';
                this.onRejectedCallbacks.forEach(fn => fn());
            }
        }
        try{//执行时可能会发生异常
            executor(resolve,reject);
        }catch (e){
            reject(e);//promise失败了
        }
       
    }
    then(onFuiFilled,onRejected){ 
        //防止值得穿透 
        onFuiFilled = typeof onFuiFilled === 'function' ? onFuiFilled : y => y;
        onRejected = typeof onRejected === 'function' ? onRejected :err => {throw err;}        
        let promise2;//作为下一次then方法的promise
       if(this.status === 'resolved'){
           promise2 = new Promise((resolve,reject) => {
               setTimeout(() => {
                  try{
                        //成功的逻辑 失败的逻辑
                        let x = onFuiFilled(this.value);
                        //看x是不是promise 如果是promise取他的结果 作为promise2成功的的结果
                        //如果返回一个普通值，作为promise2成功的结果
                        //resolvePromise可以解析x和promise2之间的关系
                        //在resolvePromise中传入四个参数，第一个是返回的promise，第二个是返回的结果，第三个和第四个分别是resolve()和reject()的方法。
                        resolvePromise(promise2,x,resolve,reject)
                  }catch(e){
                        reject(e);
                  } 
               },0)
           }); 
       } 
       if(this.status === 'rejected'){
            promise2 = new Promise((resolve,reject) => {
                setTimeout(() => {
                    try{
                        let x = onRejected(this.reason);
                        //在resolvePromise中传入四个参数，第一个是返回的promise，第二个是返回的结果，第三个和第四个分别是resolve()和reject()的方法。
                        resolvePromise(promise2,x,resolve,reject)
                    }catch(e){
                        reject(e);
                    }
                },0)

            });
       }
       //当前既没有完成也没有失败
       if(this.status === 'pending'){
           promise2 = new Promise((resolve,reject) => {
               //把成功的函数一个个存放到成功回调函数数组中
                this.onResolvedCallbacks.push( () =>{
                    setTimeout(() => {
                        try{
                            let x = onFuiFilled(this.value);
                            resolvePromise(promise2,x,resolve,reject);
                        }catch(e){
                            reject(e);
                        }
                    },0)
                });
                //把失败的函数一个个存放到失败回调函数数组中
                this.onRejectedCallbacks.push( ()=>{
                    setTimeout(() => {
                        try{
                            let x = onRejected(this.reason);
                            resolvePromise(promise2,x,resolve,reject)
                        }catch(e){
                            reject(e)
                        }
                    },0)
                })
           })
       }
       return promise2;//调用then后返回一个新的promise
    }
    catch (onRejected) {
        // catch 方法就是then方法没有成功的简写
        return this.then(null, onRejected);
    }
}
Promise.all = function (promises) {
    //promises是一个promise的数组
    return new Promise(function (resolve, reject) {
        let arr = []; //arr是最终返回值的结果
        let i = 0; // 表示成功了多少次
        function processData(index, data) {
            arr[index] = data;
            if (++i === promises.length) {
                resolve(arr);
            }
        }
        for (let i = 0; i < promises.length; i++) {
            promises[i].then(function (data) {
                processData(i, data)
            }, reject)
        }
    })
}
// 只要有一个promise成功了 就算成功。如果第一个失败了就失败了
Promise.race = function (promises) {
    return new Promise((resolve, reject) => {
        for (var i = 0; i < promises.length; i++) {
            promises[i].then(resolve,reject)
        }
    })
}
// 生成一个成功的promise
Promise.resolve = function(value){
    return new Promise((resolve,reject) => resolve(value);
}
// 生成一个失败的promise
Promise.reject = function(reason){
    return new Promise((resolve,reject) => reject(reason));
}
Promise.defer = Promise.deferred = function () {
    let dfd = {};
    dfd.promise = new Promise( (resolve, reject) =>  {
        dfd.resolve = resolve;
        dfd.reject = reject;
    });
    return dfd
}
module.exports = Promise;