如何手动封装Promise函数

第一步：Promise构造函数接受一个函数作为参数，该函数的两个参数分别是：resolve和reject;

function Promise(task) {
        // 缓存this   
        let that = this
        // 进行中的状态
        this.status = 'pending'
        //初始值
        this.value = undefined
        // 存放成功后要执行的回调函数的序列
        that.onResolvedCallbacks = []
        // 存放失败后要执行的回调函数的序列
        that.onRejectCallbacks = []
        // 该方法将Promise由pending变为fulfiled
        function resolve(value) {
            if(that.status == 'pending') {
                that.status = 'fulfiled'
                that.value = value
                that.onResolvedCallbacks.forEach(cb => cb(that.value))
            }
        }
        // 该方法是将Promise由pending变成rejected
        function reject(reason) {
            if(that.status == 'pending') {
                that.status = 'rejected'
                that.value = reason
                that.onRejectCallbacks.forEach(cb => cb(that.value))
            }
        }
        try {
            // 每一个Promise在new一个实例的时候 接受的函数都是立即执行的
            task(resolve, reject)
        } catch(e) {
            reject(e)
        }
    }

第二部 写then方法，接收两个函数onFulfilled onRejected，状态是成功态的时候调用onFulfilled 传入成功后的值，失败态的时候执行onRejected，传入失败的原因，pending 状态时将成功和失败后的这两个方法缓存到对应的数组中，当成功或失败后 依次再执行调用

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)
    })