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)
             }
          }