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.