Promise.race()

The Promise.race() method returns a promise that fulfills or rejects as soon as one of the promises in an iterable fulfills or rejects, with the value or reason from that promise.

Syntax

Promise.race(iterable);

Parameters

iterable
An iterable object, such as an Array. See iterable.

Return value

A pending Promise that asynchronously yields the value of the first promise in the given iterable to fulfill or reject.

Description

The race function returns a Promise that is settled the same way (and takes the same value) as the first promise that settles amongst the promises of the iterable passed as an argument.

If the iterable passed is empty, the promise returned will be forever pending.

If the iterable contains one or more non-promise value and/or an already settled promise, then Promise.race will resolve to the first of these values found in the iterable.

Examples

Asynchronicity of Promise.race

This following example demonstrates the asynchronicity of Promise.race:

// 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 }

An empty iterable causes the returned promise to be forever pending:

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" }

If the iterable contains one or more non-promise value and/or an already settled promise, then Promise.race will resolve to the first of these values found in the array:

var foreverPendingPromise = Promise.race([]);
var alreadyFulfilledProm = Promise.resolve(666);

var arr = [foreverPendingPromise, alreadyFulfilledProm, "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" }

Using Promise.race – examples with setTimeout

var p1 = new Promise(function(resolve, reject) { 
    setTimeout(() => resolve('one'), 500); 
});
var p2 = new Promise(function(resolve, reject) { 
    setTimeout(() => resolve('two'), 100); 
});

Promise.race([p1, p2])
.then(function(value) {
  console.log(value); // "two"
  // Both fulfill, but p2 is faster
});

var p3 = new Promise(function(resolve, reject) { 
    setTimeout(() => resolve('three'), 100);
});
var p4 = new Promise(function(resolve, reject) { 
    setTimeout(() => reject(new Error('four')), 500); 
});

Promise.race([p3, p4])
.then(function(value) {
  console.log(value); // "three"
  // p3 is faster, so it fulfills
}, function(reason) {
  // Not called
});

var p5 = new Promise(function(resolve, reject) { 
    setTimeout(() => resolve('five'), 500); 
});
var p6 = new Promise(function(resolve, reject) { 
    setTimeout(() => reject(new Error('six')), 100);
});

Promise.race([p5, p6])
.then(function(value) {
  // Not called
}, function(error) {
  console.log(error.message); // "six"
  // p6 is faster, so it rejects
});

Specifications

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  

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
race()Chrome Full support 32Edge Full support 12Firefox Full support 29IE No support NoOpera Full support 19Safari Full support 8WebView Android Full support 4.4.3Chrome Android Full support 32Firefox Android Full support 29Opera Android Full support YesSafari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12

Legend

Full support  
Full support
No support  
No support

See also