SOURCE

const PENDING = 'pending'
const RESOLVED = 'resolved'
const REJECTED = 'rejected'
class MyPromise {
    constructor(fn) {
        this.state = PENDING
        this.value = null
        this.resolveCallback = []
        this.rejectCallback = []
        const resolve = value => {
            if (this.state === PENDING) {
                this.state = RESOLVED
                this.value = value
                this.resolveCallback.map(cb => cb(value))
            }
        }
        const reject = value => {
            if (this.state === PENDING) {
                this.state = REJECTED
                this.value = value
                this.rejectCallback.map(cb => cb(value))
            }
        }
        try {
            fn(resolve, reject)
        } catch (ex) {
            reject(ex)
        }
    }
    then(onFulfilled, onRejected) {
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v
        onRejected = typeof onRejected === 'function' ? onRejected : t => {throw(t)}
        if (this.state === PENDING) {
            this.resolveCallback.push(onFulfilled)
            this.rejectCallback.push(onRejected)
        }
        if (this.state === RESOLVED) {
            onFulfilled(this.value)
        }
        if (this.state === REJECTED) {
            onRejected(this.value)
        }
    }
}

const sleep = timer => new MyPromise((resolve,reject) => setTimeout(resolve, timer))

async function AsyncF() {
    for (let i=1;i<=10;i++) {    
        await sleep(2000)
        console.log(i)
    }
}
AsyncF()