Promise.all for parallel execution in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does Promise.all do in JavaScript?

Promise.all runs multiple promises at the same time and waits for all of them to finish. It returns a new promise that resolves with an array of results when all promises succeed, or rejects if any promise fails.

beginner

How does Promise.all handle errors from promises?

If any promise inside Promise.all rejects (fails), the whole Promise.all promise immediately rejects with that error. It stops waiting for other promises.

beginner

Why use Promise.all instead of awaiting promises one by one?

Using Promise.all runs promises in parallel, so tasks happen at the same time. Awaiting one by one runs them in order, which is slower.

beginner

What is the output of Promise.all([Promise.resolve(1), Promise.resolve(2)])?

The output is a promise that resolves to [1, 2], an array of the resolved values from each promise.

intermediate

Can Promise.all be used with non-promise values?

Yes. Non-promise values are treated as resolved promises immediately. So Promise.all([1, Promise.resolve(2)]) resolves to [1, 2].

What happens if one promise in Promise.all rejects?

A The entire <code>Promise.all</code> rejects immediately.

B It waits for all promises to finish before rejecting.

C It ignores the rejected promise and resolves with others.

D It retries the rejected promise automatically.

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

Which is faster when running multiple independent promises?

A Using <code>Promise.all</code> to run them in parallel.

B Running promises inside a loop with await.

C Awaiting each promise one by one.

D Running promises synchronously without promises.

Promise.all runs promises in parallel, making it faster than awaiting one by one.

What does Promise.all([]) resolve to?

A A promise that never resolves.

B An empty array <code>[]</code> immediately.

C An error because the array is empty.

D Undefined.

Promise.all with an empty array resolves immediately with an empty array.

Can Promise.all accept a mix of promises and values?

A Only if values are strings.

B No, all items must be promises.

C Only if values are numbers.

D Yes, values are treated as resolved promises.

Non-promise values are treated as resolved promises by Promise.all.

What type of value does Promise.all return?

A An array of promises.

B A boolean.

C A new promise.

D A string.

Promise.all returns a new promise that resolves when all input promises resolve.

Explain how Promise.all helps run tasks in parallel and what happens if one task fails.

Think about waiting for all friends to finish a task together.

Describe the difference between awaiting promises one by one versus using Promise.all.

Compare waiting in line versus doing things together.