SOURCE

class MyPromise {
    // 三种状态
    static PENDING = 'pending'
    static FULFILLED = 'fulfilled'
    static REJECTED = 'rejected'
    constructor(exector) {
        this.status = MyPromise.PENDING
        this.result = null
        this.fulfilledCbs = []
        this.rejectedCbs = []
        const resolve = (value) => {
            if (this.status !== MyPromise.PENDING) return
            this.status = MyPromise.FULFILLED
            this.result = value
            while(this.fulfilledCbs.length) {
                const cb = this.fulfilledCbs.shift()
                cb()
            }
        }
        const reject = (err) => {
            if (this.status !== MyPromise.PENDING) return
            this.status = MyPromise.REJECTED
            this.result = err
            while(this.rejectedCbs.length) {
                const cb = this.rejectedCbs.shift()
                cb()
            }
        }
        exector(resolve, reject)
    }
    then(onFulfilled, onRejected) {
        return new MyPromise((resolve, reject) => {
            const handleFulfilled = () => {
                const res = onFulfilled(this.result)
                if (res instanceof MyPromise) {
                    res.then(resolve, reject)
                } else {
                    resolve(res)
                }
            }
            const handleRejected= () => {
                const err = onRejected(this.result)
                if (err instanceof MyPromise) {
                    err.then(resolve, reject)
                } else {
                    resolve(err)
                }
            }
            if (this.status === MyPromise.FULFILLED) {
                handleFulfilled()
            } else if (this.status === MyPromise.REJECTED) {
                handleRejected()
            } else {
                onFulfilled && this.fulfilledCbs.push(handleFulfilled)
                onRejected && this.rejectedCbs.push(handleRejected)
            }
        })
    }
    catch(onRejected) {
        return this.then(null, onRejected)
    }
}

const p = new MyPromise(resolve => resolve(1))
p.then(v => {
  console.log(v)
  return 100
}).then(v2 => {
  console.log(v2) // 100
})

p.then(v => {
  return new MyPromise(r => r(200))
}).then(v3 => {
  console.log(v3) //200
})

// 同一个promise多次then，互不影响
p.then(v => console.log('p1', v))
p.then(v => console.log('p2', v))