在面试过程中，要求实现promise。如果你能实现出一个简易版的Promise 既可以过关了

// 创建三个常量用于表示状态
const PENDING = 'pending'
const RESOLVED = 'resolved'
const REJECTED = 'rejected'

function MyPromise(fn) {
    const that = this
    that.state = PENDING
    that.value = null // 保存resolve 或者 reject中传入的值
    // 用于保存then中的回调 当执行完promise状态可能还是等待中，需要把then中的回调保存
    that.resolvedCallbacks = []
    that.rejectedCallbacks = []


   function resolve(value) {
       if (that.state === PENDING){
           that.state = RESOLVED
           that.value = value
           that.resolvedCallbacks.map(cb=>cb(this.value))
       }
   }
   function reject(value) {
       if (that.state === PENDING){
           that.state = REJECTED
           that.value = value
           that.rejectedCallbacks.map(cb=>cb(that.value))
       }
   }
}
MyPromise.prototype.then = function (onFulfilled,onRejected) {
    const that = this
    onFulfilled = typeof  onFulfilled === 'function' ? onFulfilled: v=>v
    onRejected = typeof  onRejected === 'function' ? onRejected : v=>v

    if (that.state === PENDING){
        that.resolvedCallbacks.push(onFulfilled)
        that.rejectedCallbacks.push(onRejected)
    }
    if (that.state === RESOLVED){
        onFulfilled(that.value)
    }
    if (that.state === REJECTED){
        onRejected(that.value)
    }
}