Lesson 20 of 80

Promise Combinators

Q: Compare Promise.all, allSettled, race, and any — what each waits for and how each handles failure.

A strong answer covers: all: waits for every promise to fulfill, rejects immediately on the first rejection; allSettled: always resolves, with the status and value/reason of every promise, never rejects; race and any both settle on the first result, but race takes the first settlement of any kind while any specifically waits for the first fulfillment (rejecting only if all reject).

Four static Promise methods for running several promises together — and they disagree sharply on what counts as 'done' and what happens when one fails.

12 min read

Check your understanding

Every option explains itself — why the right one is right, and why each wrong one is wrong. Miss one and it comes back later on a spaced-repetition schedule.

Question 1 of 2

You need to load 5 independent widgets and show whichever succeed, even if some fail. Which combinator fits?

Promise.all — it collects all the successful results and ignores failuresPromise.allSettled — it reports every outcome instead of failing the whole groupPromise.race — it returns as soon as the first widget is ready

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Interview questions on Promise Combinators

What interviewers actually ask about this, and what a strong answer has to cover.

Compare Promise.all, allSettled, race, and any — what each waits for and how each handles failure.

A strong answer covers

all: waits for every promise to fulfill, rejects immediately on the first rejection
allSettled: always resolves, with the status and value/reason of every promise, never rejects
race and any both settle on the first result, but race takes the first settlement of any kind while any specifically waits for the first fulfillment (rejecting only if all reject)

Likely follow-ups

You have 5 independent API calls and want to show whichever succeed, even if some fail — which combinator, and why not the others?
Does Promise.all cancel the other promises once one rejects? What are the implications of that?
When would Promise.any be genuinely better than Promise.race?

Practice these with the AI interviewer →

The plain-language idea

You already know a single promise settles as fulfilled or rejected. The combinators answer: given a list of promises, when is the whole group "done," and what do you get back?

const results = await Promise.all([p1, p2, p3]);
// resolves with [v1, v2, v3] IF ALL succeed
// rejects immediately with the FIRST rejection if any one fails — others keep running but their results are discarded

const results2 = await Promise.allSettled([p1, p2, p3]);
// always resolves, with [{status, value|reason}, ...] for every promise — never rejects

const first = await Promise.race([p1, p2, p3]);
// settles as soon as the FIRST promise settles, fulfilled or rejected — whichever happens first wins

const firstSuccess = await Promise.any([p1, p2, p3]);
// resolves with the first FULFILLED value; rejects only if ALL of them reject

Picking the right one

| Combinator | Waits for | Fails when | |---|---|---| | all | everything to fulfill | any one rejects | | allSettled | everything to settle | never — always tells you the outcome of each | | race | the first to settle, either way | the first settled one was a rejection | | any | the first to fulfill | every single one rejects |

all, allSettled, race, any — same inputs, different rulesStep 1 of 5

Three promises: p1 resolves fast, p2 resolves slow, p3 rejects.

What people usually miss

Watch out

all vs allSettled is the one interviewers probe hardest. If you need every result even when some fail (e.g. "load these five independent widgets, show whichever succeed"), allSettled is correct — all would throw away all the successes the moment one fails.
race isn't "the fastest wins" in a good sense necessarily — a fast rejection wins over a slow success. It's genuinely "first to settle," not "first to succeed."
any is the newest of the four and is the one actually named for "first success" — it's what people often mean when they reach for race by mistake.

Why this matters in interviews

"You have 5 independent API calls — how do you run them concurrently and handle partial failure?" is a direct real-world question, and the honest answer requires knowing which combinator actually matches "some can fail, I still want the rest."