SOURCE

function myPromise(executor) {
    let that = this
    this.status = 'pending'
    this.success = undefined
    this.error = undefined

    this.successCallBack = []
    this.errorCallBack = []

    function resolve(value) {
        if (that.status !== 'pending') return;
        that.status = 'fulfilled'
        that.success = value
        while (that.successCallBack.length)
            that.successCallBack.shift()(that.success)
    }

    function reject(err) {
        if (that.status !== 'pending') return;
        that.status = 'rejected'
        that.error = err
        while (that.errorCallBack.length)
            that.errorCallBack.shift()(err)
    }

    try {
        executor(resolve, reject)
    } catch (err) {
        reject(err)
    }
}

myPromise.prototype.then = function (resolve, reject) {
    let that = this
    let pro = new myPromise((resolve1, reject1) => {
        if (that.status === 'pending') {
            this.successCallBack.push(() => {
                let val = resolve(that.success)
                resolve1(val)
            })
            this.errorCallBack.push(() => {
                let err = reject(that.error)
                reject1(err)
            })
        }
        else if (that.status === 'fulfilled') {
            let val = resolve(that.success)
            resolve1(val)
        }
        else {
            let err = reject(that.error)
            reject1(err)
        }
    })
    return pro
}
let p1 = new myPromise((resolve, reject) => {
    resolve('123')
})
console.log(p1)
p1.then(function (value) {
        console.log('第一个成功回调')
    }, function () { })
    .then(function () {
        console.log('第二个成功回调')
    }, function () { })
    .then(function () {}, 
    function () { console.log('第三个成功回调') })