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Promise.all vs allSettled vs race vs any (JavaScript Promise Combinators)Frontend interview answer

HighIntermediateJavascript

By FrontendAtlas Team · Updated Apr 10, 2026

Interview focus

This JavaScript interview question tests whether you can explain Promise combinators: all vs allSettled vs race vs any in production, connect it to production trade-offs, and handle common follow-up questions.

Promise combinators: all vs allSettled vs race vs any in production explanation without falling back to memorized docs wording
Promise and Async reasoning, edge cases, and production failure modes
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Interview quick answer

Promise combinators coordinate multiple async operations. all fails fast, allSettled always returns results, race resolves/rejects with the first settled promise, and any resolves with the first fulfilled promise (or rejects with AggregateError if all reject). Production answers map each one to partial-failure UIs, timeout guards, and fallback endpoints.

Full interview answer

The mental model

All four take an iterable of promises (values are auto-wrapped via Promise.resolve). The key differences are:

1) What “finishes” the combinator (first failure? first settle? first success? all done?)
2) What it returns (values only vs per-promise status objects)
3) How errors behave (fail-fast vs aggregate)

Interviewers usually want one real use case per combinator: all for dependencies, allSettled for partial-failure UIs, race for timeout guards, and any for fallback mirrors. Also: these combinators do not cancel the other promises; they just decide what result to produce.

Combinator	Resolves when...	Rejects when...	Resolved value shape	Typical use
Promise.all	All promises fulfill	Any promise rejects (fail-fast)	Array of fulfillment values (in input order)	All-or-nothing dependencies (e.g., load config + user + flags)
Promise.allSettled	All promises settle (fulfilled or rejected)	Never (unless input iteration throws)	Array of result objects: `{status:'fulfilled', value}` or `{status:'rejected', reason}`	Collect outcomes (e.g., show partial results, log failures)
Promise.race	First promise settles and it fulfills	First promise settles and it rejects	The first settled value (or rejection reason)	Timeouts / “first response wins”
Promise.any	First promise fulfills	All promises reject	The first fulfilled value; rejects with `AggregateError`	Fallbacks (e.g., first successful endpoint/CDN)

Cheat sheet: finish condition + error semantics.

Minimal examples

JAVASCRIPT

const ok1 = Promise.resolve('A');
const ok2 = Promise.resolve('B');
const bad = Promise.reject(new Error('X'));

// 1) all: fails fast on first rejection
await Promise.all([ok1, ok2]);          // -> ['A', 'B']
await Promise.all([ok1, bad, ok2]);     // -> rejects with Error('X')

// 2) allSettled: always returns status objects
await Promise.allSettled([ok1, bad, ok2]);
// -> [
//   { status: 'fulfilled', value: 'A' },
//   { status: 'rejected', reason: Error('X') },
//   { status: 'fulfilled', value: 'B' }
// ]

// 3) race: first settle wins (could be reject)
await Promise.race([bad, ok1]);         // -> rejects (bad settles first)
await Promise.race([ok1, bad]);         // -> 'A'

// 4) any: first fulfillment wins (ignores rejects until all reject)
await Promise.any([bad, ok2]);          // -> 'B'
await Promise.any([bad, Promise.reject('Y')]);
// -> rejects with AggregateError (contains all reasons)

Important details interviewers look for

1) Input order: all and allSettled keep the output array aligned with the input order, not completion order.

2) Fail-fast vs keep-going:

all rejects as soon as one rejects.

allSettled waits for all.

race settles on the first settled promise (resolve or reject).

any resolves on the first fulfillment; only rejects if everything rejects.

3) No cancellation: Promise.all rejecting does not stop the other operations. If you need cancellation, you need explicit support (e.g., AbortController for fetch).

4) AggregateError: Promise.any rejects with an AggregateError, which bundles all rejection reasons (often via error.errors in many runtimes).

Classic pattern: timeout with race

Use race to bound time. But remember: the original async work continues unless it’s cancellable.

JAVASCRIPT

function timeout(ms) {
  return new Promise((_, reject) =>
    setTimeout(() => reject(new Error('Timeout')), ms)
  );
}

async function fetchWithTimeout(url, ms, signal) {
  // Prefer real cancellation (AbortController) when possible.
  const fetchPromise = fetch(url, { signal });
  return Promise.race([fetchPromise, timeout(ms)]);
}

Classic pattern: first successful response with any

JAVASCRIPT

async function firstHealthy(urls) {
  return Promise.any(
    urls.map(async (u) => {
      const r = await fetch(u);
      if (!r.ok) throw new Error(`${u} -> ${r.status}`);
      return r;
    })
  );
}

// If all endpoints fail -> AggregateError

JAVASCRIPT

try {
  await Promise.any([mirrorA(), mirrorB(), mirrorC()]);
} catch (error) {
  if (error instanceof AggregateError) {
    console.error(error.errors); // every rejection reason
  }
}

Decision shortcut

Promise.all for “I need every dependency before render”.
Promise.allSettled for dashboards or batch jobs where partial failure is still useful.
Promise.race for timeout guards or first-settled control flow.
Promise.any for mirrors, fallbacks, or “first healthy response wins”.

One-liners to memorize

all: “All succeed or I fail fast.”

allSettled: “Tell me how each one ended.”

race: “First one to settle decides.”

any: “First success wins; otherwise aggregate failure.”