From 218934fa2ed1c702a6d3923d2aa2cc6b43c48684 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:43:23 -0500 Subject: initial commit --- .../global_objects/promise/race/index.html | 171 +++++++++++++++++++++ 1 file changed, 171 insertions(+) create mode 100644 files/zh-tw/web/javascript/reference/global_objects/promise/race/index.html (limited to 'files/zh-tw/web/javascript/reference/global_objects/promise/race/index.html') diff --git a/files/zh-tw/web/javascript/reference/global_objects/promise/race/index.html b/files/zh-tw/web/javascript/reference/global_objects/promise/race/index.html new file mode 100644 index 0000000000..9f0d8b4d2e --- /dev/null +++ b/files/zh-tw/web/javascript/reference/global_objects/promise/race/index.html @@ -0,0 +1,171 @@ +--- +title: Promise.race() +slug: Web/JavaScript/Reference/Global_Objects/Promise/race +translation_of: Web/JavaScript/Reference/Global_Objects/Promise/race +--- +
{{JSRef}}
+ +

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。

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語法

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Promise.race(iterable);
+ +

參數

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+
iterable
+
一個 iterable 物件，像是 {{jsxref("Array")}}. 請參考可迭代協議。
+
+ +

回傳值

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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 {{jsxref("Promise")}}。

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描述

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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。

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範例

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Promise.race 的非同步性質

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以下例子演示了 Promise.race 的非同步性質：

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// we are passing as argument an array of promises that are already resolved,
+// to trigger Promise.race as soon as possible
+var resolvedPromisesArray = [Promise.resolve(33), Promise.resolve(44)];
+
+var p = Promise.race(resolvedPromisesArray);
+// immediately logging the value of p
+console.log(p);
+
+// using setTimeout we can execute code after the stack is empty
+setTimeout(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：

+ +
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：

+ +
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

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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

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SpecificationStatusComment
{{SpecName('ES2015', '#sec-promise.race', 'Promise.race')}}{{Spec2('ES2015')}}Initial definition in an ECMA standard.
{{SpecName('ESDraft', '#sec-promise.race', 'Promise.race')}}{{Spec2('ESDraft')}} 
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瀏覽器相容性

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{{Compat("javascript.builtins.Promise.race")}}

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參見

+ + -- cgit v1.2.3-54-g00ecf