Promise.race and Promise.allSettled in Node.js

Introduction

Promise.race and Promise.allSettled help you handle multiple tasks that happen at the same time. They let you decide what to do when some or all tasks finish.

You want to get the result of the fastest task among many.

You want to wait for all tasks to finish, no matter if they succeed or fail.

You want to handle multiple requests and see which one finishes first.

You want to know the outcome of every task without stopping on errors.

You want to run several tasks and continue only after all have settled.

Syntax

Node.js

Promise.race(iterableOfPromises)
Promise.allSettled(iterableOfPromises)

Promise.race returns a promise that settles as soon as one of the promises settles (either fulfilled or rejected).

Promise.allSettled returns a promise that resolves after all promises have settled, with an array of their results.

Examples

Returns the result of the first promise that finishes, whether it succeeds or fails.

Node.js

Promise.race([promise1, promise2, promise3])

Waits for all promises to finish and gives you an array showing which succeeded or failed.

Node.js

Promise.allSettled([promise1, promise2, promise3])

Sample Program

This code creates three promises: one resolves after 300ms, one rejects after 200ms, and one resolves after 400ms.

Promise.race returns the first promise that settles, which is the rejected one after 200ms.

Promise.allSettled waits for all promises to finish and then shows their status and values or reasons.

Node.js

const promise1 = new Promise(resolve => setTimeout(() => resolve('First done'), 300));
const promise2 = new Promise((_, reject) => setTimeout(() => reject('Second failed'), 200));
const promise3 = new Promise(resolve => setTimeout(() => resolve('Third done'), 400));

// Using Promise.race
Promise.race([promise1, promise2, promise3])
  .then(result => console.log('Race result:', result))
  .catch(error => console.log('Race error:', error));

// Using Promise.allSettled
Promise.allSettled([promise1, promise2, promise3])
  .then(results => {
    console.log('AllSettled results:');
    results.forEach((res, i) => {
      console.log(`Promise ${i + 1}:`, res.status, res.status === 'fulfilled' ? res.value : res.reason);
    });
  });

Important Notes

Promise.race settles as soon as any promise settles, so it can reject or resolve first.

Promise.allSettled never rejects; it always resolves with the status of all promises.

Use Promise.allSettled when you want to know the outcome of all promises without stopping on errors.

Summary

Promise.race gives you the first finished promise result or error.

Promise.allSettled waits for all promises and tells you how each ended.

Both help manage multiple tasks running at the same time in a simple way.

Practice

1. What does Promise.race do when given multiple promises?

easy

A. Cancels all promises except the first one.

B. Waits for all promises to finish and returns their results.

C. Returns only the results of promises that resolved successfully.

D. Returns the result or error of the first promise that finishes.

Solution

1. Step 1: Understand Promise.race behavior

   Promise.race returns as soon as any promise settles (resolves or rejects).

2. Step 2: Compare with other Promise methods

   Unlike Promise.all or allSettled, it does not wait for all promises.

3. Final Answer:

   Returns the result or error of the first promise that finishes. -> Option D

4. Quick Check:

   Promise.race = first finished promise result [OK]

Hint: Remember race means first to finish wins [OK]

Common Mistakes:

• Thinking it waits for all promises
• Assuming it only returns successful results
• Believing it cancels other promises

2. Which of the following is the correct syntax to use Promise.allSettled with an array of promises named tasks?

easy

A. Promise.allSettled(tasks).finally(() => console.log('done'));

B. Promise.allSettled(tasks).catch(error => console.log(error));

C. Promise.allSettled(tasks).then(results => console.log(results));

D. Promise.allSettled(tasks).resolve(results => console.log(results));

Solution

1. Step 1: Recall Promise.allSettled usage

   Promise.allSettled returns a promise that resolves with an array of results.

2. Step 2: Check correct method chaining

   We use .then() to handle the resolved results, not .catch() or .resolve().

3. Final Answer:

   Promise.allSettled(tasks).then(results => console.log(results)); -> Option C

4. Quick Check:

   Use .then() to get allSettled results [OK]

Hint: Use .then() to handle Promise.allSettled results [OK]

Common Mistakes:

• Using .catch() instead of .then() for results
• Trying to use .resolve() method on promise
• Assuming .finally() receives results

3. Consider the code:

const p1 = new Promise(res => setTimeout(() => res('A'), 100));
const p2 = new Promise((_, rej) => setTimeout(() => rej('Error'), 50));
Promise.race([p1, p2])
  .then(console.log)
  .catch(console.error);

What will be printed?

medium

A. 'Error'

B. 'A'

C. An array of results

D. Nothing, code throws syntax error

Solution

1. Step 1: Identify which promise settles first

   p2 rejects after 50ms, p1 resolves after 100ms, so p2 finishes first.

2. Step 2: Understand Promise.race behavior on rejection

   Promise.race rejects immediately with the first rejection, so .catch logs 'Error'.

3. Final Answer:

   'Error' -> Option A

4. Quick Check:

   First finished promise is rejection 'Error' [OK]

Hint: Check which promise settles first, resolve or reject [OK]

Common Mistakes:

• Assuming resolve wins over reject
• Expecting an array instead of single result
• Thinking code throws syntax error

4. What is wrong with this code snippet?

const promises = [Promise.resolve(1), Promise.reject('fail')];
Promise.allSettled(promises).catch(console.error);

medium

A. Promise.allSettled requires async/await syntax.

B. Promise.allSettled never rejects, so .catch will never run.

C. Promises array must contain only resolved promises.

D. You must use .then() before .catch() with allSettled.

Solution

1. Step 1: Recall Promise.allSettled behavior

   It waits for all promises and never rejects, it always resolves with results.

2. Step 2: Understand .catch usage here

   Since it never rejects, .catch will never be called, so error handling is ineffective.

3. Final Answer:

   Promise.allSettled never rejects, so .catch will never run. -> Option B

4. Quick Check:

   allSettled always resolves, .catch unused [OK]

Hint: allSettled never rejects, so .catch is useless here [OK]

Common Mistakes:

• Thinking allSettled rejects on any promise failure
• Believing async/await is required
• Assuming .then() must come before .catch() always

5. You want to run three tasks: task1, task2, and task3. You want to get the first task that finishes successfully, but if all fail, you want to know all errors. Which approach correctly achieves this?

hard

A. Use Promise.race on all tasks, then if it rejects, run Promise.allSettled to get all errors.

B. Use Promise.allSettled first, then pick the first successful result from the array.

C. Use Promise.all on all tasks and catch errors to get all results.

D. Use Promise.race and ignore errors from rejected promises.

Solution

1. Step 1: Understand requirement for first success or all errors

   Promise.race gives first finished promise, but may reject if first is failure.

2. Step 2: Combine Promise.race and Promise.allSettled

   If Promise.race rejects, then run Promise.allSettled to collect all errors from all tasks.

3. Step 3: Evaluate other options

   Promise.allSettled alone waits for all, no early success; Promise.all fails fast; ignoring errors loses info.

4. Final Answer:

   Use Promise.race on all tasks, then if it rejects, run Promise.allSettled to get all errors. -> Option A

5. Quick Check:

   race first success, allSettled for all errors [OK]

Hint: Race first success, fallback to allSettled for errors [OK]

Common Mistakes:

• Using allSettled only and waiting too long
• Ignoring rejected promises in race
• Using all which fails on first rejection