手寫promise

function Promise(task) {
    let that = this;//緩存this
    //默認狀態爲pending
    that.status = 'pending';
    //此變量裏放着此promise的結果
    that.value = undefined;
    //存放的着全部成功的回調函數
    that.onResolvedCallbacks = [];
    //存放着全部的失敗的回調函數
    that.onRejectedCallbacks = [];
    //調用此方法能夠把promise變成成功態
    //resolve的時候你把掙到的錢傳進來
    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));
        }
    }

    //調用此方法能夠把當前的promise變成失敗態
    function reject(reason) {
        //若是當前狀態是初始態，則轉成失敗態
        if (that.status == 'pending') {
            that.status = 'rejected';
            that.value = reason;
            that.onRejectedCallbacks.forEach(item=>item(that.value));
        }
    }

    //當即執行傳入的任務
    try {
        task(resolve, reject);
    } catch (e) {
        reject(e);
    }
}
function resolvePromise(promise2,x,resolve,reject){
    let then;
    //若是x就是promise2
    if(promise2 === x){
        return reject(new TypeError('循環引用'));
    }
    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成功的回調，onReject失敗的回調
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;