手动实现 Promise-优快云博客

本文链接：https://blog.youkuaiyun.com/weixin_43899407/article/details/133060015

本文介绍了如何从基础出发实现一个简单的Promise类，包括构造函数、then方法和静态resolve/reject方法，以及Promise的三种状态和回调处理。重点展示了Promise的异步操作管理和错误处理机制。

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实现：

  1 ((root, factory) => {
  2     if (typeof exports === "object") {
  3         module.exports = factory();
  4     } else {
  5         root.MyPromise = factory();
  6     }
  7 })(this, function () {
  8     const PENDING = "pending",
  9         FULFILLED = "fulfilled",
 10         REJECTED = "rejected";
 11 
 12     return class MyPromise {
 13         status = PENDING;
 14         data = null;
 15         onResolvedCache = null;
 16         onRejectedCache = null;
 17         constructor(executor) {
 18             const [resolve, reject] = [
 19                 (value) => {
 20                     if (this.status === PENDING) {
 21                         this.data = value;
 22                         this.status = FULFILLED;
 23                         this.onResolvedCache && this.onResolvedCache();
 24                     }
 25                 },
 26                 (reason) => {
 27                     if (this.status === PENDING) {
 28                         this.data = reason;
 29                         this.status = REJECTED;
 30                         this.onRejectedCache && this.onRejectedCache();
 31                     }
 32                 },
 33             ];
 34 
 35             try {
 36                 executor(resolve, reject);
 37             } catch (err) {
 38                 reject(err);
 39             }
 40         }
 41 
 42         then(onResolved, onRejected) {
 43             return new MyPromise((resolve, reject) => {
 44                 const handle = (callback) => {
 45                     let result = null;
 46                     if (callback instanceof Function) {
 47                         result = callback(this.data);
 48                     }
 49 
 50                     if (result instanceof MyPromise) {
 51                         result.then(resolve, reject);
 52                     } else {
 53                         this.status === FULFILLED
 54                             ? resolve(this.data)
 55                             : reject(this.data);
 56                     }
 57                 };
 58 
 59                 switch (this.status) {
 60                     case PENDING:
 61                         this.onResolvedCache = () => handle(onResolved);
 62                         this.onRejectedCache = () => handle(onRejected);
 63                         break;
 64                     case FULFILLED:
 65                         handle(onResolved);
 66                         break;
 67                     case REJECTED:
 68                         handle(onRejected);
 69                         break;
 70                 }
 71             });
 72         }
 73 
 74         catch(onRejected) {
 75             return this.then(null, onRejected);
 76         }
 77 
 78         static resolve(data) {
 79             return new MyPromise((resolve, reject) => {
 80                 if (data instanceof MyPromise) {
 81                     data.then(resolve, reject);
 82                 } else {
 83                     resolve(data);
 84                 }
 85             });
 86         }
 87 
 88         static reject(reason) {
 89             return new MyPromise((resolve, reject) => {
 90                 if (reason instanceof MyPromise) {
 91                     reason.then(resolve, reject);
 92                 } else {
 93                     reject(reason);
 94                 }
 95             });
 96         }
 97 
 98         static all(promises) {
 99             return new MyPromise((resolve, reject) => {
100                 let values = [];
101                 let resolvedCount = 0;
102                 const promiseTotal = promises.length;
103                 promises.forEach((promise) => {
104                     promise.then(
105                         (res) => {
106                             values.push(res);
107                             resolvedCount++;
108                             if (resolvedCount === promiseTotal) {
109                                 resolve(values);
110                             }
111                         },
112                         (err) => {
113                             reject(err);
114                         },
115                     );
116                 });
117             });
118         }
119 
120         static race(promises) {
121             return new MyPromise((resolve, reject) => {
122                 promises.forEach((promise) => {
123                     promise.then(
124                         (res) => {
125                             resolve(res);
126                         },
127                         (err) => {
128                             reject(err);
129                         },
130                     );
131                 });
132             });
133         }
134     };
135 });