Promise.race and Promise.allSettled in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does Promise.race do in JavaScript?

Promise.race returns a new promise that settles as soon as any of the input promises settles (either resolves or rejects). It "races" all promises and settles with the first one that finishes.

intermediate

What is the difference between Promise.allSettled and Promise.all?

Promise.allSettled waits for all promises to settle (resolve or reject) and returns their results regardless of success or failure. Promise.all waits for all to resolve but rejects immediately if any promise rejects.

beginner

What does the result of Promise.allSettled look like?

It returns an array of objects, each with a status property ("fulfilled" or "rejected") and either a value (for fulfilled) or reason (for rejected).

beginner

If you want to get the first promise that finishes regardless of success or failure, which method do you use?

You use Promise.race because it settles as soon as any promise settles, whether it resolves or rejects.

beginner

Can Promise.allSettled be used to handle multiple promises without stopping on errors?

Yes, Promise.allSettled lets you wait for all promises to finish and see which succeeded or failed without stopping early due to errors.

What does Promise.race return?

A The last promise to resolve

B An array of all resolved values

C An array of all rejected reasons

D The result of the first promise to settle (resolve or reject)

Promise.race settles as soon as the first promise settles, returning its result or error.

Which method waits for all promises to finish regardless of success or failure?

A Promise.race

B Promise.allSettled

C Promise.all

D Promise.any

Promise.allSettled waits for all promises to settle, no matter if they resolve or reject.

What happens if one promise rejects in Promise.all?

A It immediately rejects with that error

B It ignores the rejection

C It waits for all promises anyway

D It returns an array of results

Promise.all rejects immediately if any promise rejects.

What property indicates a promise was fulfilled in Promise.allSettled results?

A status: 'rejected'

B reason: 'fulfilled'

C status: 'fulfilled'

D value: 'fulfilled'

The status property is either 'fulfilled' or 'rejected'.

Which method would you use to get the fastest successful promise result, ignoring failures?

A Promise.any

B Promise.allSettled

C Promise.race

D Promise.all

Promise.any returns the first fulfilled promise, ignoring rejected ones.

Explain how Promise.race works and give a simple example.

Think about a race where the first to finish wins.

Describe the purpose of Promise.allSettled and how its results are structured.

It tells you how every promise ended, no matter what.

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