SOURCE

class MyPromise {
    constructor(fn) {
        this.state = 'pending'
        this.resolveVal = null
        this.rejectVal = null
        this.resolveCbArr = []
        this.rejectCbArr = []
        fn(this.resolve.bind(this), this.reject.bind(this))
    }
    resolve(val) {
        if (this.state === 'pending') {
            this.state = 'resolve'
            this.resolveVal = val
            this.resolveCbArr.map(cb => cb(this.resolveVal))
        }
    }
    reject(val) {
        if (this.state === 'pending') {
            this.state = 'reject'
            this.rejectVal = val
            this.rejectCbArr.map(cb => cb(this.rejectVal))
        }
    }
    then(resolveCb, rejectCb) {
        if(this.state === 'pending') {
            this.resolveCbArr.push(resolveCb)
            this.rejectCbArr.push(rejectCb)
        }
        if (this.state === 'resolve') {
            resolveCb(this.resolveVal)
        }
        if (this.state === 'reject') {
            rejectCb(this.rejectVal)
        }
    }
}

const p1 = new MyPromise((resolve, reject) => {
    setTimeout(() => {
        resolve(111)
    }, 2000)
}).then(res => {
    console.log(res)
}, () => {

})