Promise.race and Promise.allSettled in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to use Promise.race to get the first resolved promise.

Node.js

Promise.[1]([promise1, promise2]).then(result => console.log(result));

Drag options to blanks, or click blank then click option'

AallSettled

Ball

Crace

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.all instead of Promise.race, which waits for all promises.

Using Promise.allSettled which waits for all promises to settle.

Promise.race returns a promise that resolves or rejects as soon as one of the promises in the iterable resolves or rejects.

2fill in blank

medium

Complete the code to use Promise.allSettled to wait for all promises to settle.

Node.js

Promise.[1]([promiseA, promiseB]).then(results => console.log(results));

Drag options to blanks, or click blank then click option'

Aall

Brace

Cany

DallSettled

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.all which rejects immediately if any promise rejects.

Using Promise.race which returns the first settled promise only.

Promise.allSettled waits for all promises to settle, regardless of resolve or reject, and returns their results.

3fill in blank

hard

Fix the error in the code to correctly handle the first settled promise.

Node.js

Promise.[1]([p1, p2, p3]).then(value => console.log('First:', value));

Drag options to blanks, or click blank then click option'

AallSettled

Brace

Call

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.all which waits for all promises to resolve.

Using Promise.allSettled which returns all results, not just the first.

To get the first settled promise's value, use Promise.race. Other methods behave differently.

4fill in blank

hard

Fill both blanks to create a Promise.allSettled usage that logs each promise's status and value.

Node.js

Promise.[1]([pA, pB]).then(results => {
  results.forEach(result => {
    console.log(result.[2] + ':', result.value || result.reason);
  });
});

Drag options to blanks, or click blank then click option'

AallSettled

Brace

Cstatus

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.all which rejects on first failure.

Trying to access 'value' without checking status.

Promise.allSettled returns an array of objects with status and either value or reason. Logging status shows if each promise fulfilled or rejected.

5fill in blank

hard

Fill all three blanks to create a Promise.race that handles the first promise and logs its result or error.

Node.js

Promise.[1]([pX, pY, pZ])
  .then(result => console.log('Resolved:', result))
  .catch(error => console.log('Rejected:', error));

Drag options to blanks, or click blank then click option'

Arace

BallSettled

Call

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using Promise.all which waits for all promises.

Not handling rejection with catch.

Promise.race returns a promise that settles as soon as one of the promises settles, either resolved or rejected. The then handles success, and catch handles rejection.

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