1：結構的設計

主要思路：

建構函式 Class----就可以new來呼叫 利用constructor實現函式的呼叫（executor) 實現呼叫了reject()resolve無效呼叫 ---記錄Promise的狀態  const PEOMISE_STATUS_PENDING='pending'  預設狀態  const PEOMISE_STATUS_FULFILLED='fulfilled'  const PEOMISE_STATUS_REJECTED='rejected' if判斷是某狀態就執行某回撥 考慮傳遞引數 儲存引數預設undefined 2：then方法 在構造的類裡面寫一個then函式 執行then傳入進來的回撥函式 巧妙運用定時器來調整程式碼執行順序----->queueMicrotask(規範)

 1
 
 const PEOMISE_STATUS_PENDING = 'pending'
 2 const PEOMISE_STATUS_FULFILLED = 'fulfilled'
 3 const PEOMISE_STATUS_REJECTED = 'rejected'
 4 class tyyPromise {
 5     constructor(executor) {
 6         this.status = PEOMISE_STATUS_PENDING
 7         this.value = undefined
 8         this.reason = undefined
 
 9         const resolve = (value) => {
10             if (this.status === PEOMISE_STATUS_PENDING) {
11                 this.status = PEOMISE_STATUS_FULFILLED
12                 queueMicrotask(() => {
13                     this.value = value
14                     this.onFulfilled(this.value);
 
15                 })
16             }
17 
18 
19         }
20         const reject = (reason) => {
21             if (this.status === PEOMISE_STATUS_PENDING) {
22                 this.status = PEOMISE_STATUS_REJECTED
23                 //以免then還沒執行
24                 queueMicrotask(() => {
25                     this.reason = reason
26                     this.onFulfilled(this.reason);
27                 })
28             }
29         }
30 
31         executor(resolve, reject)
32     }
33     then(onFulfilled, onRejected) {
34         this.onFulfilled = onFulfilled
35         this.onRejected = onRejected
36 
37     }
38 }
39 const promise = new tyyPromise((resolve, reject) => {
40     // console.log('pending');
41     reject(111)
42     resolve(222)
43 
44 })
45 promise.then(res => {
46     console.log('res:', res);
47 }, err => {
48     console.log('err:', err);
49 })

 

 這樣就能簡單的實現Promise的基本功能

但是有待完善 比如 多次呼叫then都會執行，鏈式呼叫等等