简单手写js实现promise

在JavaScript中，Promise是一种处理异步操作的技术，它使得我们可以更方便地编写非阻塞的、链式调用的代码。下面是一个简单的手动实现Promise的例子： ```javascript // 定义Promise构造函数 function Promise(executor) { // 将executor函数封装到resolve和reject函数中 var resolve = function(value) { if (this.status === 'fulfilled' || this.status === 'rejected') { throw new Error('Promise already settled'); } this.value = value; this.status = 'fulfilled'; this.onFulfilled.forEach(function(onFulfillment) { try { onFulfillment(value); } catch(e) { this.reject(e); } }, this); }; var reject = function(reason) { if (this.status === 'fulfilled' || this.status === 'rejected') { throw new Error('Promise already settled'); } this.reason = reason; this.status = 'rejected'; this.onerror.forEach(function(onError) { onError(reason); }, this); }; // 初始化状态和回调队列 this.status = 'pending'; this.value = undefined; this.reason = undefined; // 执行 executor 函数并返回 executor(resolve, reject); } // 实现基本的方法 .then() 和 .catch() Promise.prototype.then = function(onFulfillment, onRejected) { var promise = new Promise(function(resolve, reject) { this.onFulfilled = onFulfillment ? [onFulfillment] : []; this.onerror = onRejected ? [onRejected] : []; resolve(this.value); }); if (this.status === 'rejected') { promise.reject(this.reason); } return promise; }; Promise.prototype.catch = function(onRejected) { return this.then(null, onRejected); }; // 使用示例 var myPromise = new Promise(function(resolve, reject) { setTimeout(function() { resolve('Task completed successfully!'); }, 2000); }); myPromise .then(function(result) { console.log(result); // 输出 "Task completed successfully!" after 2 seconds }) .catch(function(error) { console.error(error); }); ```