Promise.race()

The Promise.race() method takes an iterable of promises as input and returns a single Promise. This returned promise settles with the eventual state of the first promise that settles.

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Syntax

Promise.race(iterable)

Parameters

iterable

An iterable (such as an Array) of promises.

Return value

A Promise that asynchronously settles with the eventual state of the first promise in the iterable to settle. In other words, it fulfills if the first promise to settle is fulfilled, and rejects if the first promise to settle is rejected. The returned promise remains pending forever if the iterable passed is empty. If the iterable passed is non-empty but contains no pending promises, the returned promise is still asynchronously (instead of synchronously) settled.

Description

The Promise.race() method is one of the promise concurrency methods. It's useful when you want the first async task to complete, but do not care about its eventual state (i.e. it can either succeed or fail).

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

Examples

Using Promise.race()

This example shows how Promise.race() can be used to race several timers implemented with setTimeout(). The timer with the shortest time always wins the race and becomes the resulting promise's state.

function sleep(time, value, state) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (state === "fulfill") {
        return resolve(value);
      } else {
        return reject(new Error(value));
      }
    }, time);
  });
}

const p1 = sleep(500, "one", "fulfill");
const p2 = sleep(100, "two", "fulfill");

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

const p3 = sleep(100, "three", "fulfill");
const p4 = sleep(500, "four", "reject");

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

const p5 = sleep(500, "five", "fulfill");
const p6 = sleep(100, "six", "reject");

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

Asynchronicity of Promise.race

This following example demonstrates the asynchronicity of Promise.race. Unlike other promise concurrency methods, Promise.race is always asynchronous: it never settles synchronously, even when the iterable is empty.

// Passing an array of promises that are already resolved,
// to trigger Promise.race as soon as possible
const resolvedPromisesArray = [Promise.resolve(33), Promise.resolve(44)];

const 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(() => {
  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:

const foreverPendingPromise = Promise.race([]);
console.log(foreverPendingPromise);
setTimeout(() => {
  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 settle to the first of these values found in the array:

const foreverPendingPromise = Promise.race([]);
const alreadyFulfilledProm = Promise.resolve(100);

const arr = [foreverPendingPromise, alreadyFulfilledProm, "non-Promise value"];
const arr2 = [foreverPendingPromise, "non-Promise value", Promise.resolve(100)];
const p = Promise.race(arr);
const p2 = Promise.race(arr2);

console.log(p);
console.log(p2);
setTimeout(() => {
  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() to implement request timeout

You can race a potentially long-lasting request with a timer that rejects, so that when the time limit has elapsed, the resulting promise automatically rejects.

const data = Promise.race([
  fetch("/api"),
  new Promise((resolve, reject) => {
    // Reject after 5 seconds
    setTimeout(() => reject(new Error("Request timed out")), 5000);
  }),
])
  .then((res) => res.json())
  .catch((err) => displayError(err));

If the data promise fulfills, it will contain the data fetched from /api; otherwise, it will reject if fetch remains pending for 5 seconds and loses the race with the setTimeout timer.

Using Promise.race() to detect the status of a promise

Because Promise.race() resolves to the first non-pending promise in the iterable, we can check a promise's state, including if it's pending. This example is adapted from promise-status-async.

function promiseState(promise) {
  const pendingState = { status: "pending" };

  return Promise.race([promise, pendingState]).then(
    (value) =>
      value === pendingState ? value : { status: "fulfilled", value },
    (reason) => ({ status: "rejected", reason }),
  );
}

In this function, if promise is pending, the second value, pendingState, which is a non-promise, becomes the result of the race; otherwise, if promise is already settled, we may know its state through the onFulfilled and onRejected handlers. For example:

const p1 = new Promise((res) => setTimeout(() => res(100), 100));
const p2 = new Promise((res) => setTimeout(() => res(200), 200));
const p3 = new Promise((res, rej) => setTimeout(() => rej(300), 100));

async function getStates() {
  console.log(await promiseState(p1));
  console.log(await promiseState(p2));
  console.log(await promiseState(p3));
}

console.log("Immediately after initiation:");
getStates();
setTimeout(() => {
  console.log("After waiting for 100ms:");
  getStates();
}, 100);

// Logs:
// Immediately after initiation:
// { status: 'pending' }
// { status: 'pending' }
// { status: 'pending' }
// After waiting for 100ms:
// { status: 'fulfilled', value: 100 }
// { status: 'pending' }
// { status: 'rejected', reason: 300 }

Note: The promiseState function still runs asynchronously, because there is no way to synchronously get a promise's value (i.e. without then() or await), even when it is already settled. However, promiseState() always fulfills within one tick and never actually waits for any promise's settlement.

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.error("failed with reason:", reason);
  });
// 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.error("failed with reason:", reason);
  });
// succeeded with value: one

Specifications

Specification
ECMAScript Language Specification
# sec-promise.race

Browser compatibility

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See also