如何手動封裝Promise函數

第一步：Promise構造函數接受一個函數作為參數，該函數的兩個參數分別是：resolve和reject;

function Promise(task) {
        // 緩存this   
        let that = this
        // 進行中的狀態
        this.status = 'pending'
        //初始值
        this.value = undefined
        // 存放成功后要執行的回調函數的序列
        that.onResolvedCallbacks = []
        // 存放失敗后要執行的回調函數的序列
        that.onRejectCallbacks = []
        // 該方法將Promise由pending變為fulfiled
        function resolve(value) {
            if(that.status == 'pending') {
                that.status = 'fulfiled'
                that.value = value
                that.onResolvedCallbacks.forEach(cb => cb(that.value))
            }
        }
        // 該方法是將Promise由pending變成rejected
        function reject(reason) {
            if(that.status == 'pending') {
                that.status = 'rejected'
                that.value = reason
                that.onRejectCallbacks.forEach(cb => cb(that.value))
            }
        }
        try {
            // 每一個Promise在new一個實例的時候 接受的函數都是立即執行的
            task(resolve, reject)
        } catch(e) {
            reject(e)
        }
    }

第二部 寫then方法，接收兩個函數onFulfilled onRejected，狀態是成功態的時候調用onFulfilled 傳入成功后的值，失敗態的時候執行onRejected，傳入失敗的原因，pending 狀態時將成功和失敗后的這兩個方法緩存到對應的數組中，當成功或失敗后 依次再執行調用

Promise.prototype.then = function(onFulfilled, onRejected) {
        if(this.status == 'fulfiled') {
            onFulfilled(this.value)
        }
        if(this.status == 'rejected') {
            onRejected(this.value)
        }
        if (this.status == 'pending') {
            this.onResolvedCallbacks.push(onFulfilled(this.value));
            this.onRejectCallbacks.push(onRejected(this.value));
        }
    }
    new Promise(function (resolve, reject) {
        resolve(100);// reject(100)
    }).then(function (data) {
        console.log('666', data)
    })