此頁面由社群從英文翻譯而來。了解更多並加入 MDN Web Docs 社群。
Promise.race()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
Promise.race(iterable) 方法回傳一個 promise 物件，此 promise 物件會於 iterable 引數中任一個 promise 轉為 resolve 或 rejected 時立即轉變成 resolve 或 rejected，並且接收其成功值或失敗訊息。
語法
js
Promise.race(iterable);參數
回傳值
當傳入的 iterable 中有 promise 被實現或拒絕時，立刻回傳被實現或拒絕的Promise。
描述
race 函式回傳一個與傳入的 iterable 之中第一個被解決（settled）的 promise 相同方式被解決（且以相同值）的Promise。
範例
Promise.race 的非同步性質
以下例子演示了Promise.race 的非同步性質：
js
// we are passing as argument an array of promises that are already resolved,// to trigger Promise.race as soon as possiblevar resolvedPromisesArray = [Promise.resolve(33), Promise.resolve(44)];var p = Promise.race(resolvedPromisesArray);// immediately logging the value of pconsole.log(p);// using setTimeout we can execute code after the stack is emptysetTimeout(function () { console.log("the stack is now empty"); console.log(p);});// logs, in order:// Promise { <state>: "pending" }// the stack is now empty// Promise { <state>: "fulfilled", <value>: 33 }一個空的 iterable 造成回傳的 promise 永久擱置：
js
var foreverPendingPromise = Promise.race([]);console.log(foreverPendingPromise);setTimeout(function () { console.log("the stack is now empty"); console.log(foreverPendingPromise);});// logs, in order:// Promise { <state>: "pending" }// the stack is now empty// Promise { <state>: "pending" }若 iterable 中有一個或多個非 promise 值且／或一個已經被實現／解決的 promise，Promise.race 將以陣列中第一個這樣的值解決：
js
var foreverPendingPromise = Promise.race([]);var alreadyResolvedProm = Promise.resolve(666);var arr = [foreverPendingPromise, alreadyResolvedProm, "non-Promise value"];var arr2 = [foreverPendingPromise, "non-Promise value", Promise.resolve(666)];var p = Promise.race(arr);var p2 = Promise.race(arr2);console.log(p);console.log(p2);setTimeout(function () { console.log("the stack is now empty"); console.log(p); console.log(p2);});// logs, in order:// Promise { <state>: "pending" }// Promise { <state>: "pending" }// the stack is now empty// Promise { <state>: "fulfilled", <value>: 666 }// Promise { <state>: "fulfilled", <value>: "non-Promise value" }使用Promise.race 及setTimeout 的範例
js
var p1 = new Promise(function (resolve, reject) { setTimeout(resolve, 500, "one");});var p2 = new Promise(function (resolve, reject) { setTimeout(resolve, 100, "two");});Promise.race([p1, p2]).then(function (value) { console.log(value); // "two" // Both resolve, but p2 is faster});var p3 = new Promise(function (resolve, reject) { setTimeout(resolve, 100, "three");});var p4 = new Promise(function (resolve, reject) { setTimeout(reject, 500, "four");});Promise.race([p3, p4]).then( function (value) { console.log(value); // "three" // p3 is faster, so it resolves }, function (reason) { // Not called },);var p5 = new Promise(function (resolve, reject) { setTimeout(resolve, 500, "five");});var p6 = new Promise(function (resolve, reject) { setTimeout(reject, 100, "six");});Promise.race([p5, p6]).then( function (value) { // Not called }, function (reason) { console.log(reason); // "six" // p6 is faster, so it rejects },);規範
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-promise.race |
瀏覽器相容性
Loading…