// 定義異步調用的主類，名為 MyPromise
class MyPromise {// 執行器接收 resolve 和 reject 方法來改變 promise 的狀態constructor(executor) {// 初始化狀態為 "pending"this.state = "pending";// 初始化值為 undefinedthis.value = undefined;// 初始化原因為 undefinedthis.reason = undefined;// 完成狀態回調函數集this.onResolvedCallbacks = [];// 拒絕狀態回調函數集this.onRejectedCallbacks = [];// 在resolve中為value賦值const resolve = (value) => {//當狀態為pending時進行狀態更改和賦值操作if (this.state === "pending") {this.state = "fulfilled";this.value = value;// 當promise從pending狀態轉為fulfilled，會執行onResolvedCallbacks中的所有回調函數this.onResolvedCallbacks.forEach((fn) => fn());}};// 在reject中為reason賦值const reject = (reason) => {// 當狀態為 pending 時進行狀態更改和賦值操作if (this.state === "pending") {this.state = "rejected";this.reason = reason;// 當promise從pending狀態轉為rejected，會執行onRejectedCallbacks中的所有回調函數this.onRejectedCallbacks.forEach((fn) => fn());}};// 立即執行executor，錯誤則執行rejecttry {executor(resolve, reject);} catch (err) {reject(err);}}// promise 有一個名為 then 的方法，它接受兩個參數：onFulfilled 和 onRejected，然后返回一個新的 promisethen(onFulfilled, onRejected) {// 對處理函數進行判定，如果其為空或者其不是函數，則賦值為默認函數onFulfilled =typeof onFulfilled === "function" ? onFulfilled : (value) => value;onRejected =typeof onRejected === "function"? onRejected: (reason) => {throw reason;};// 返回新的promise對象const promise2 = new MyPromise((resolve, reject) => {if (this.state === "fulfilled") {setTimeout(() => {try {const x = onFulfilled(this.value);this.resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}}, 0);}if (this.state === "rejected") {setTimeout(() => {try {const x = onRejected(this.reason);this.resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}}, 0);}// 當state為pending時，將onFulfilled和onRejected存放起來if (this.state === "pending") {this.onResolvedCallbacks.push(() => {setTimeout(() => {try {const x = onFulfilled(this.value);this.resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}}, 0);});this.onRejectedCallbacks.push(() => {setTimeout(() => {try {const x = onRejected(this.reason);this.resolvePromise(promise2, x, resolve, reject);} catch (err) {reject(err);}}, 0);});}});return promise2;}// 對不同情況的x進行處理resolvePromise(promise2, x, resolve, reject) {// promise 和 x 引用同一對象，錯誤處理if (promise2 === x) {return reject(new TypeError("Chaining cycle detected for promise"));}let called = false;if (x instanceof MyPromise) {x.then((value) => {if (called) return;called = true;this.resolvePromise(promise2, value, resolve, reject);},(reason) => {if (called) return;called = true;reject(reason);});} else {resolve(x);}}// 返回一個完成狀態的promisestatic resolve(value) {return new MyPromise((resolve, reject) => {resolve(value);});}// 返回一個拒絕狀態的promisestatic reject(reason) {return new MyPromise((resolve, reject) => {reject(reason);});}// 返回一個 Promise 實例，該實例在 iterable 參數內所有的 promise 都“完成（Fulfilled）”或參數中不包含 promise 時回調完成（resolve）static all(promises) {return new MyPromise((resolve, reject) => {const result = [];let count = 0;const processResult = (index, value) => {result[index] = value;count++;// 所有結果都得到了則更新狀態為fulfilledif (count === promises.length) {resolve(result);}};// 使用 forEach，能遍歷到不是promise的其他類型promises.forEach((promise, index) => {if (promise instanceof MyPromise) {promise.then((value) => processResult(index, value),(reason) => reject(reason));} else {processResult(index, promise);}});});}// 返回一個 Promise 實例，只有當 iterable 參數內的某個 promise 成功或失敗時，返回的 promise 才會異步地 success 或 failurestatic race(promises) {return new MyPromise((resolve, reject) => {promises.forEach((promise) => {if (promise instanceof MyPromise) {promise.then(resolve, reject);} else {resolve(promise);}});});}
}

調用舉例

// 創建一個 Promise 對象
const promise = new MyPromise((resolve, reject) => {// 異步操作，比如網絡請求或者定時器setTimeout(() => {// 異步操作成功，調用 resolve 并傳遞結果resolve('成功');// 或者異步操作失敗，調用 reject 并傳遞錯誤原因// reject('失敗');}, 1000);
});// 使用 then 方法添加成功和失敗的回調函數
promise.then(value => {console.log('成功的結果：', value);},reason => {console.log('失敗的原因：', reason);}
);