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

Promise.race(iterable);

參數

iterable
一個 iterable 物件,像是 Array. 請參考可迭代協議

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:

// 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

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 Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Promise.race' in that specification.
Standard Initial definition in an ECMA standard.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Promise.race' in that specification.
Draft  

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