SOURCE

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

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

    _resolve = value => {
        if (this.status === PENDING) {
            this.status = FULFILLED
            this.value = value
        }
    }

    _reject = reason => {
        if (this.status === PENDING) {
            this.status = REJECTED
            this.value = reason
        }
    }

    then(onFulfilled, onRejected) {
        const { status, value } = this
        if (status === FULFILLED) {
            onFulfilled(value)
        }
        if (status === REJECTED) {
            onRejected(value)
        }
    }
}

new VPromise((resolve, reject) => {
    say('hello world!')
}).then(res => {
    console.log(1, res)
}, err => {
    console.log(2, err)
})