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
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(100);
var arr = [foreverPendingPromise, alreadyFulfilledProm, "non-Promise value"];
var arr2 = [foreverPendingPromise, "non-Promise value", Promise.resolve(100)];
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>: 100 }
// 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(error) {
// 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
});
Comparison with Promise.any
Promise.race
takes the first settled Promise
.
const promise1 = new Promise((resolve, reject) => {
setTimeout(resolve, 500, 'one');
});
const promise2 = new Promise((resolve, reject) => {
setTimeout(reject, 100, 'two');
});
Promise.race([promise1, promise2]).then((value) => {
console.log('succeeded with value:', value);
}).catch((reason) => {
// Only promise1 is fulfilled, but promise2 is faster
console.log('failed with reason:', reason);
});
// expected output: "failed with reason: two"
Promise.any
takes the first fulfilled Promise
.
const promise1 = new Promise((resolve, reject) => {
setTimeout(resolve, 500, 'one');
});
const promise2 = new Promise((resolve, reject) => {
setTimeout(reject, 100, 'two');
});
Promise.any([promise1, promise2]).then((value) => {
// Only promise1 is fulfilled, even though promise2 settled sooner
console.log('succeeded with value:', value);
}).catch((reason) => {
console.log('failed with reason:', reason);
});
// expected output: "succeeded with value: one"
Specifications
Specification |
---|
ECMAScript Language Specification # sec-promise.race |
Browser compatibility
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