SOURCE

class MyPromise {
    constructor(executor) {
        this.initBind()
        this.initValue()

        try {
            executor(this.resolve, this.reject)
        } catch (err) {
            this.reject(err)
        }
    }

    initBind() {
        // 初始化this
        this.resolve = this.resolve.bind(this)
        this.reject = this.reject.bind(this)
    }

    initValue() {
        this.PromiseState = 'pending'
        this.PromiseResult = null
        this.onFulfilledCallbacks = []
        this.onRejectedCallbacks = []
    }

    resolve(value) {
        if (this.PromiseState !== 'pending') return
        this.PromiseState = 'fulfilled'
        this.PromiseResult = value

        while (this.onFulfilledCallbacks.length) {
            this.onFulfilledCallbacks.shift()(this.PromiseResult)
        }
    }

    reject(err) {
        if (this.PromiseState !== 'pending') return
        this.PromiseState = 'rejected'
        this.PromiseResult = err

        while (this.onRejectedCallbacks.length) {
            this.onRejectedCallbacks.shift()(this.PromiseResult)
        }
    }

    then(onFulfilled, onRejected) {
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : val => val;
        onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };

        var thenPromise = new MyPromise((resolve, reject) => {
            const resolvePromise = cb => {
                try {
                    const x = cb(this.PromiseResult)
                    if (x instanceof MyPromise) {
                        x.then(resolve, reject)
                    } else {
                        resolve(x)
                    }
                } catch(err) {
                    reject(err)
                }
            }
            if (this.PromiseState === 'fulfilled') {
                resolvePromise(onFulfilled)
            } else if (this.PromiseState === 'rejected') {
                resolvePromise(onRejected)
            } else if (this.PromiseState === 'pending') {
                this.onFulfilledCallbacks.push(onFulfilled.bind(this))
                this.onRejectedCallbacks.push(onRejected.bind(this))
            }

        })


        return thenPromise
    }
}

var test = new MyPromise((resolve, reject) => {
    console.log('construct')
    resolve('hello')
    // setTimeout(() => {
    //     resolve('hellow')
    // }, 500)
    // reject('hellow2')
    // throw('3')
})

test.then((res) => {
    console.log('first then: ', res)
    return res + ' world'
}).then(res => {
    console.log('sec then: ', res)
    return new MyPromise((resolve, reject) => {
        resolve(res + ', javascript')
    }).then(res => {
        console.log('then promise', res)
        return res + ' then'
    })
}).then(res => {
    console.log('third then: ', res)
})

// console.log('test', test)