SOURCE

class myPromise{
    static PENDING = 'pending'
    static FULLFILLED = 'resolved'
    static REJECTED = 'rejected'
    constructor (executor) {
        this.status = myPromise.PENDING
        this.value = undefined
        this.reason = undefined
        // 成功时的回调
        this.onResolvedCbs = []
        // 失败时的回调
        this.onRejectedCbs = []
        const resolve =  (value) => {
            if (this.status === myPromise.PENDING) {
                this.status = myPromise.FULLFILLED
                this.value = value
                this.onResolvedCbs.forEach(fun => fun.call())
            }
        }
        const reject = (error) => {
            if (this.status === myPromise.PENDING) {
                this.status = myPromise.REJECTED
                this.reason = error
                this.onRejectedCbs.forEach(fun => fun())
            }
        }
        try{
            executor(resolve, reject)
        } catch (e) {
            reject(e)
        }
    }

    then (onFulFilled = function () {}, onRejected = function () {}) {
        console.log(this)
        let value = this.value
        // 根据Promise实例状态决定then的执行
        return new myPromise((resolve, reject) => {
            let value
            if (this.status === myPromise.PENDING) {
            this.onResolvedCbs.push(() => {
                setTimeout(() => {
                    try {
                        value = onFulFilled(this.value) || value
                        resolve(value) 
                    } catch (e) {
                        reject(e)
                    }
                })
            })
            this.onRejectedCbs.push(() => {
                setTimeout(() => {
                    try {
                        value = onRejected(this.reason)
                        resolve(value || lastValue) 
                    } catch (e) {           
                        reject(e)
                    }
                })

            })
        } else if (this.status === myPromise.FULLFILLED) {
            setTimeout(() => {
                try {
                    value = onFulFilled(this.value) || value
                    resolve(value)
                } catch (e) {
                    reject(e)
                }
            })

        } else {
                try {
                    onRejected(this.reason)
                } catch (e) {
                    reject(e)
                }
        }
        })
    }
}

const p = new myPromise((resolve, reject) => {
    resolve('hello')
}).then().then((value) => {
    console.log(value)
    return 'hi'
    }).then(val => console.log(val), err => console.log(err.toString()))