手写promise

摘要：缓存默认状态为此变量里放着此的结果存放的着所有成功的回调函数存放着所有的失败的回调函数调用此方法可以把变成成功态的时候你把挣到的钱传进来调用此方法可以把当前的变成失败态如果当前状态是初始态，则转成失败态立即执行传入的任务如果就是循环引用成功

function Promise(task) {
    let that = this;//缓存this
    //默认状态为pending
    that.status = "pending";
    //此变量里放着此promise的结果
    that.value = undefined;
    //存放的着所有成功的回调函数
    that.onResolvedCallbacks = [];
    //存放着所有的失败的回调函数
    that.onRejectedCallbacks = [];
    //调用此方法可以把promise变成成功态
    //resolve的时候你把挣到的钱传进来
    function resolve(value) {
        if(value instanceof Promise){
            return value.then(resolve,reject);
        }
        if (that.status == "pending") {
            that.status = "fulfilled";
            that.value = value;
            that.onResolvedCallbacks.forEach(item=>item(that.value));
        }
    }

    //调用此方法可以把当前的promise变成失败态
    function reject(reason) {
        //如果当前状态是初始态，则转成失败态
        if (that.status == "pending") {
            that.status = "rejected";
            that.value = reason;
            that.onRejectedCallbacks.forEach(item=>item(that.value));
        }
    }

    //立即执行传入的任务
    try {
        task(resolve, reject);
    } catch (e) {
        reject(e);
    }
}
function resolvePromise(promise2,x,resolve,reject){
    let then;
    //如果x就是promise2
    if(promise2 === x){
        return reject(new TypeError("循环引用"));
    }
    if(x instanceof Promise){
        if(x.status == "pending"){
            x.then(function(y){
                resolvePromise(promise2,y,resolve,reject);
            },reject);
        }else if(x.status == "fulfilled"){
            resolve(x.value);
        }else if(x.status == "rejected"){
            reject(x.value);
        }
    }else if(x!=null && (typeof x == "object" || typeof x == "function")){
        try{
            then = x.then;
            if(typeof then == "function"){
                then.call(x,function(y){
                    resolvePromise(promise2,y,resolve,reject)
                },reject);
            }
        }catch(e){
            reject(e);
        };
    }else{
        resolve(x);
    }
}
//onFulfilled成功的回调，onReject失败的回调
Promise.prototype.then = function (onFulfilled, onReject) {
    onFulfilled = typeof onFulfilled == "function"?onFulfilled:function(value){return value};
    onReject = typeof onReject=="function"?onReject:function(reason){
        throw reason;
    }
    let that = this;
    let promise2;
    if(that.status == "fulfilled"){
        promise2 = new Promise(function(resolve,reject){
            let x = onFulfilled(that.value);
            resolvePromise(promise2,x,resolve,reject);
        });
    }
    if(that.status == "rejected"){
        promise2 = new Promise(function(resolve,reject){
            let x = onReject(that.value);
            resolvePromise(promise2,x,resolve,reject);
        });

    }
    if(that.status == "pending"){
        promise2 = new Promise(function(resolve,reject){
            that.onResolvedCallbacks.push(function(){
                let x = onFulfilled(that.value);
                resolvePromise(promise2,x,resolve,reject);
            });
            that.onRejectedCallbacks.push(function(){
                let x = onReject(that.value);
                resolvePromise(promise2,x,resolve,reject);
            });
        });
    }
    return promise2;
}

Promise.all = function (arr) {
    return new Promise((resolve,reject) =>{
        let values = []
        let len = arr.length
        for(var i = 0;i < len; i++){
            let promise = arr[i]
            if(typeof promise.then == "function"){
                promise.then(res=>{
                    values.push(res)
                    if(values.length == len){
                        resolve(values)
                    }
                })
            }
        }
    })
}

Promise.race = function (arr) {
    return new Promise((resolve,reject) =>{
        let len = arr.length
        for(var i = 0;i < len; i++){
            let promise = arr[i]
            if(typeof promise.then == "function"){
                promise.then(res=>{
                    resolve(res)
                })
            }
        }
    })
}

Promise.resolve = function (value) {
    if (value instanceof Promise) return value
    return new Promise(resolve => resolve(value))
}

Promise.reject = function (value) {
    return new Promise((resolve,reject)=>reject(value))
}


module.exports = Promise;