Promise.all, allSettled, race, and any: A Complete Guide

Promise.all, allSettled, race, and any: A Complete Guide

JavaScript has four promise combinators. Each has a different behavior. Here is each one.

Promise.all (All Must Succeed)

const [a, b, c] = await Promise.all([fetchA(), fetchB(), fetchC()]);

• Runs all in parallel.
• Waits for all to fulfill.
• If any rejects, the whole thing rejects (fast fail).
• Results are in order.

Promise.allSettled (Wait for All, Even Failures)

const results = await Promise.allSettled([fetchA(), fetchB(), fetchC()]);
results.forEach((r) => {
  if (r.status === "fulfilled") console.log(r.value);
  else console.error(r.reason);
});

• Runs all in parallel.
• Waits for all to settle (fulfilled or rejected).
• Never rejects. Returns [{ status, value/reason }].

Promise.race (First to Settle Wins)

const result = await Promise.race([fetchA(), fetchB()]);

• Runs all in parallel.
• Returns as soon as the first promise settles (fulfilled or rejected).
• If the first to settle fulfills, the race fulfills.
• If the first to settle rejects, the race rejects.

Promise.any (First Success Wins)

const result = await Promise.any([fetchA(), fetchB(), fetchC()]);
``
- Runs all in parallel.
- Returns as soon as the first promise **fulfills**.
- If all reject, rejects with `AggregateError`.

### Comparison

| Combinator | Waits For | Rejects If | Returns |
|---|---|---|---|
| `Promise.all` | All fulfill | Any rejects | Array of values |
| `Promise.allSettled` | All settle | Never | Array of { status, value/reason } |
| `Promise.race` | First settles | First rejects | First value or error |
| `Promise.any` | First fulfills | All reject | First value (or AggregateError) |

### The Takeaway

`Promise.all` (all must succeed), `allSettled` (all settle, partial success), `race` (first to settle, timeouts), `any` (first success, all-fail returns `AggregateError`). Use the right combinator for your use case.