SOURCE

const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class VPromise {
    constructor(excutor) {
        this.status = PENDING
        this.value = undefined
        this.fulfilledQueues = []
        this.rejectedQueues = []

        try {
            excutor(this._resolve, this._reject)
        } catch (e) {
            this._reject(e)
        }
    }

    _resolve = value => {
        if (this.status === PENDING) {
            this.status = FULFILLED
            this.value = value
            let cb
            while (cb = this.fulfilledQueues.shift()) {
                cb(value)
            }
        }
    }

    _reject = reason => {
        if (this.status === PENDING) {
            this.status = REJECTED
            this.value = reason
            let cb
            while (cb = this.rejectedQueues.shift()) {
                cb(reason)
            }
        }
    }

    then(onFulfilled, onRejected) {
        const { status, value, fulfilledQueues, rejectedQueues } = this
        if (status === FULFILLED) {
            onFulfilled(value)
        }
        if (status === REJECTED) {
            onRejected(value)
        }
        // 执行器是异步的情况，把回调放到回调队列里
        if (status === PENDING) {
            fulfilledQueues.push(onFulfilled)
            rejectedQueues.push(onRejected)
        }
    }
}

new VPromise(function (resolve, reject) {
    setTimeout(() => {
        resolve('aa')
    }, 1000)
}).then(res => {
    console.log(1, res) // 1秒后打印： 1 "aa"
}, err => {
    console.log(2, err)
})