Promise.all for parallel execution in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Promise.all example?

Consider the following Node.js code using Promise.all. What will be logged to the console?

Node.js

const p1 = Promise.resolve(10);
const p2 = new Promise(resolve => setTimeout(() => resolve(20), 100));
const p3 = Promise.resolve(30);

Promise.all([p1, p2, p3]).then(results => console.log(results));

A [10, 30, 20]

B [30, 20, 10]

C Error: Promise rejected

D [10, 20, 30]

💡 Hint

Promise.all waits for all promises to resolve and keeps the order of the input array.

Promise.all returns an array of results in the same order as the promises were passed, regardless of which resolves first.

📝 Syntax

intermediate

Which option correctly uses Promise.all to fetch data?

You want to fetch data from three URLs in parallel using Promise.all. Which code snippet is correct?

A const results = Promise.all(fetch(url1), fetch(url2), fetch(url3));

B const results = await Promise.all([fetch(url1), fetch(url2), fetch(url3)]);

C const results = await Promise.all(fetch(url1), fetch(url2), fetch(url3));

D const results = Promise.all([fetch(url1), fetch(url2), fetch(url3)]).then();

💡 Hint

Promise.all expects a single array of promises.

Promise.all requires an array of promises. Option B correctly awaits the array of fetch promises.

🔧 Debug

advanced

Why does this Promise.all code never resolve?

Look at this code snippet. Why does the Promise.all never resolve or reject?

Node.js

const p1 = new Promise(() => {});
const p2 = Promise.resolve(2);

Promise.all([p1, p2]).then(console.log).catch(console.error);

A p1 never resolves or rejects, so Promise.all waits forever.

B p2 rejects immediately causing Promise.all to hang.

C The .then callback is missing, so no output occurs.

D Promise.all requires at least three promises to resolve.

💡 Hint

A promise that never settles blocks Promise.all from finishing.

Promise.all waits for all promises to settle. Since p1 never settles, Promise.all never resolves or rejects.

❓ state_output

advanced

What is the value of 'results' after this Promise.all call?

Given the code below, what will be the value of 'results' inside the then callback?

Node.js

const p1 = Promise.resolve('a');
const p2 = Promise.resolve('b');
const p3 = Promise.resolve('c');

Promise.all([p1, p2, p3]).then(results => {
  // What is results?
  console.log(results);
});

A ['a', 'b', 'c']

B ['c', 'b', 'a']

C ['a', 'c', 'b']

D undefined

💡 Hint

Promise.all preserves the order of promises in the input array.

The results array matches the order of the input promises regardless of resolution timing.

🧠 Conceptual

expert

Which option best describes Promise.all behavior when one promise rejects?

What happens when one promise in Promise.all rejects while others are still pending?

A Promise.all waits for all promises to settle before rejecting.

B Promise.all resolves with all fulfilled values and ignores rejections.

C Promise.all immediately rejects with that error, ignoring other promises.

D Promise.all retries the rejected promise until it resolves.

💡 Hint

Promise.all rejects as soon as any promise rejects.

Promise.all rejects immediately when any promise rejects, with that rejection reason.

Practice

1. What does Promise.all do in Node.js?

easy

A. Runs promises one after another in sequence

B. Runs only the first promise and ignores others

C. Runs multiple promises in parallel and waits for all to complete

D. Runs promises but returns results in random order

Solution

1. Step 1: Understand Promise.all behavior

   Promise.all runs all promises at the same time (in parallel) and waits until all finish.

2. Step 2: Check result order and completion

   It returns results in the order of the promises given, not random or sequentially one by one.

3. Final Answer:

   Runs multiple promises in parallel and waits for all to complete -> Option C

4. Quick Check:

   Promise.all = parallel run + wait all [OK]

Hint: Promise.all runs all promises together, not one by one [OK]

Common Mistakes:

• Thinking Promise.all runs promises sequentially
• Believing Promise.all returns results in random order
• Assuming Promise.all ignores failed promises

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

easy

A. Promise.all(promises).then(results => console.log(results));

B. Promise.all.then(promises).catch(error => console.log(error));

C. Promise.all(promises).catch(results => console.log(results));

D. Promise.all(promises).finally(results => console.log(results));

Solution

1. Step 1: Check Promise.all usage

   Promise.all is called as a function with an array of promises as argument.

2. Step 2: Verify chaining with then()

   To get results, use .then() after Promise.all to handle resolved values.

3. Final Answer:

   Promise.all(promises).then(results => console.log(results)); -> Option A

4. Quick Check:

   Correct syntax = Promise.all(array).then() [OK]

Hint: Use Promise.all(array).then() to get results [OK]

Common Mistakes:

• Using Promise.all.then(promises) instead of Promise.all(promises).then()
• Using catch() to handle results instead of errors
• Using finally() to get results instead of then()

3. What will be logged by this code?

const p1 = Promise.resolve(1);
const p2 = Promise.resolve(2);
const p3 = Promise.resolve(3);

Promise.all([p1, p2, p3]).then(results => console.log(results));

medium

A. Error: Promise rejected

B. [1, 2, 3]

C. [undefined, undefined, undefined]

D. [3, 2, 1]

Solution

1. Step 1: Understand resolved promises

   p1, p2, p3 are promises resolved immediately with values 1, 2, and 3.

2. Step 2: Promise.all returns array of results in input order

   Promise.all waits for all to resolve and returns results in the same order as input array.

3. Final Answer:

   [1, 2, 3] -> Option B

4. Quick Check:

   Promise.all results order = input order [OK]

Hint: Promise.all returns results in same order as input promises [OK]

Common Mistakes:

• Assuming results order depends on resolution speed
• Expecting reversed or random order
• Thinking Promise.all returns undefined values

4. Identify the error in this code snippet:

const p1 = Promise.resolve('A');
const p2 = Promise.reject('Error');

Promise.all([p1, p2])
  .then(results => console.log('Results:', results))
  .catch(error => console.log('Caught:', error));

medium

A. The catch block will run with 'Error' because p2 rejects

B. Promise.all will never reject even if one promise fails

C. The then block will run with partial results

D. Syntax error: Promise.reject cannot be used inside Promise.all

Solution

1. Step 1: Check behavior of Promise.all with rejected promises

   If any promise rejects, Promise.all immediately rejects with that error.

2. Step 2: Analyze catch and then blocks

   Since p2 rejects, the catch block runs with the error message 'Error'. The then block does not run.

3. Final Answer:

   The catch block will run with 'Error' because p2 rejects -> Option A

4. Quick Check:

   Promise.all rejects if any promise rejects [OK]

Hint: If one promise rejects, Promise.all rejects immediately [OK]

Common Mistakes:

• Thinking Promise.all ignores rejected promises
• Expecting then block to run with partial results
• Believing Promise.reject causes syntax error here

5. You want to fetch data from three APIs in parallel and process all results only if all succeed. Which code correctly uses Promise.all to achieve this?

const fetch1 = () => fetch('https://api1.example.com/data').then(res => res.json());
const fetch2 = () => fetch('https://api2.example.com/data').then(res => res.json());
const fetch3 = () => fetch('https://api3.example.com/data').then(res => res.json());

// Which code snippet correctly waits for all fetches and handles errors?

hard

A. Promise.all([fetch1(), fetch2(), fetch3()])
.then(results => console.error('All data:', results))
.catch(error => console.log('Fetch failed:', error));

B. Promise.all([fetch1, fetch2, fetch3])
.then(results => console.log('All data:', results))
.catch(error => console.error('Fetch failed:', error));

C. Promise.all(fetch1, fetch2, fetch3)
.then(results => console.log('All data:', results))
.catch(error => console.error('Fetch failed:', error));

D. Promise.all([fetch1(), fetch2(), fetch3()])
.then(results => console.log('All data:', results))
.catch(error => console.error('Fetch failed:', error));

Solution

1. Step 1: Check how to call fetch functions

   fetch1, fetch2, fetch3 are functions returning promises, so call them with () to get promises.

2. Step 2: Verify Promise.all usage and error handling

   Pass an array of promises to Promise.all, then use .then() to handle results and .catch() for errors.

3. Final Answer:

   Promise.all([fetch1(), fetch2(), fetch3()]) .then(results => console.log('All data:', results)) .catch(error => console.error('Fetch failed:', error)); -> Option D

4. Quick Check:

   Call functions to get promises, pass array to Promise.all [OK]

Hint: Call functions to get promises before Promise.all [OK]

Common Mistakes:

• Passing functions instead of calling them
• Passing multiple arguments instead of an array
• Swapping console.log and console.error in then/catch