实现⼀个简洁版的promise

// 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

             this.state = PENDING

             // value 变量⽤于保存 resolve 或者 reject 中传⼊的值

             this.value = null

             // ⽤于保存 then 中的回调，因为当执⾏完 Promise 时状态可能还是等待中，这时候应该把

             that.resolvedCallbacks = []

             that.rejectedCallbacks = []

             function resolve(value) {

                 // ⾸先两个函数都得判断当前状态是否为等待中

                 if(that.state === PENDING) {

                     that.state = RESOLVED

                     that.value = value

                     // 遍历回调数组并执⾏

                     that.resolvedCallbacks.map(cb=>cb(that.value))

                 }

             }

             function reject(value) {

                if(that.state === PENDING) {

                     that.state = REJECTED

                     that.value = value

                     that.rejectedCallbacks.map(cb=>cb(that.value))

                 }

             }

             // 完成以上两个函数以后，我们就该实现如何执⾏ Promise 中传⼊的函数了

             try {

                 fn(resolve,reject)

             }cach(e){

                 reject(e)

             }

          }

          // 最后我们来实现较为复杂的 then 函数

          MyPromise.prototype.then = function(onFulfilled,onRejected){

             const that = this

             // 判断两个参数是否为函数类型，因为这两个参数是可选参数

             onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v=>v

             onRejected = typeof onRejected === 'function' ? onRejected : e=>throw e

             // 当状态不是等待态时，就去执⾏相对应的函数。如果状态是等待态的话，就往回调函数中 push

             if(this.state === PENDING) {

               this.resolvedCallbacks.push(onFulfilled)

               this.rejectedCallbacks.push(onRejected)

             }

             if(this.state === RESOLVED) {

               onFulfilled(that.value)

             }

             if(this.state === REJECTED) {

               onRejected(that.value)

             }

          }

实现⼀个简洁版的promise的相关教程结束。