function MyPromise(executor) { this.status = 'pending'; // 初始状态 this.value = undefined; // 存放异步操作的结果 this.reason = undefined; // 存放错误原因 this.onFulfilledCallbacks = []; // 存放成功的回调函数 this.onRejectedCallbacks = []; // 存放失败的回调函数 // 更改 Promise 的状态 function resolve(value) { if (this.status === 'pending') { this.status = 'fulfilled'; this.value = value; this.onFulfilledCallbacks.forEach(fn => fn()); } } function reject(reason) { if (this.status === 'pending') { this.status = 'rejected'; this.reason = reason; this.onRejectedCallbacks.forEach(fn => fn()); } } // 捕获 executor 执行器中的错误 try { executor(resolve.bind(this), reject.bind(this)); } catch (e) { reject.call(this, e); } // 实现 then 方法 MyPromise.prototype.then = function(onFulfilled, onRejected) { onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value; onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason; }; return new MyPromise((resolve, reject) => { if (this.status === 'fulfilled') { setTimeout(() => { try { let x = onFulfilled(this.value); resolvePromise(resolve, reject, x); } catch (e) { reject(e); } }); } else if (this.status === 'rejected') { setTimeout(() => { try { let x = onRejected(this.reason); resolvePromise(resolve, reject, x); } catch (e) { reject(e); } }); } else { this.onFulfilledCallbacks.push(() => { setTimeout(() => { try { let x = onFulfilled(this.value); resolvePromise(resolve, reject, x); } catch (e) { reject(e); } }); }); this.onRejectedCallbacks.push(() => { setTimeout(() => { try { let x = onRejected(this.reason); resolvePromise(resolve, reject, x); } catch (e) { reject(e); } }); }); } }); }; // 辅助函数，处理 then 或 catch 中的返回值 function resolvePromise(resolve, reject, x) { // ... 省略了一些复杂的处理逻辑，如处理 x 为 Promise 的情况 if (x !== promise2) { // 避免自引用 if (x instanceof MyPromise) { x.then(resolve, reject); } else { resolve(x); } } } }