Promise.race and Promise.allSettled in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does Promise.race output?

Consider the following code snippet using Promise.race. What will be logged to the console?

Node.js

const p1 = new Promise(resolve => setTimeout(() => resolve('First'), 300));
const p2 = new Promise(resolve => setTimeout(() => resolve('Second'), 100));
Promise.race([p1, p2]).then(result => console.log(result));

A Promise pending

B "First"

C undefined

D "Second"

💡 Hint

Think about which promise resolves first based on the timeout durations.

Promise.race returns the result of the first promise that settles (resolves or rejects). Since p2 resolves after 100ms and p1 after 300ms, the output is "Second".

❓ state_output

intermediate

What is the output of Promise.allSettled?

Given the following promises, what will Promise.allSettled return?

Node.js

const p1 = Promise.resolve('Success');
const p2 = Promise.reject('Error');
Promise.allSettled([p1, p2]).then(results => console.log(results));

A Promise rejected with 'Error'

B [{status: 'fulfilled', value: 'Success'}, {status: 'fulfilled', value: 'Error'}]

C [{status: 'fulfilled', value: 'Success'}, {status: 'rejected', reason: 'Error'}]

D [{status: 'rejected', reason: 'Success'}, {status: 'rejected', reason: 'Error'}]

💡 Hint

Remember that allSettled waits for all promises and reports their status.

Promise.allSettled returns an array describing each promise's outcome. The first promise fulfilled with value "Success" and the second rejected with reason "Error".

📝 Syntax

advanced

Which option causes a syntax error with Promise.race?

Identify which code snippet will cause a syntax error when using Promise.race.

A Promise.race(Promise.resolve(1), Promise.resolve(2)).then(console.log);

B Promise.race([Promise.resolve(1), Promise.reject(2)]).then(console.log);

C Promise.race([Promise.resolve(1), Promise.resolve(2)]).then(console.log);

D Promise.race([]).then(console.log);

💡 Hint

Check the expected argument type for Promise.race.

Promise.race expects a single iterable (like an array) of promises. Option A passes two arguments instead of one array, causing a syntax error.

🔧 Debug

advanced

Why does this Promise.allSettled code not log results?

This code does not log anything. What is the reason?

Node.js

const p1 = Promise.resolve('Done');
const p2 = Promise.reject('Fail');
Promise.allSettled([p1, p2]);

A Missing .then() to handle the results

B Promise.allSettled does not work with rejected promises

C Promises must be awaited with async/await

D The array passed is empty

💡 Hint

Promises are lazy until you handle their result.

Without .then() or await, the promise settles but no code runs to log the results.

🧠 Conceptual

expert

Which statement about Promise.race and Promise.allSettled is true?

Select the only true statement about Promise.race and Promise.allSettled:

A Promise.allSettled resolves as soon as the first promise settles

B Promise.race resolves or rejects as soon as any promise settles

C Promise.race waits for all promises to settle before resolving

D Promise.allSettled rejects if any promise rejects

💡 Hint

Think about how each method handles multiple promises and their timing.

Promise.race settles as soon as any promise settles (resolves or rejects). Promise.allSettled waits for all promises to settle and never rejects.

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