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.