Error handling in parallel requests comes down to what should happen when one fails.

Semantics first

Pattern: independent best-effort

const results = await Promise.allSettled(loaders);

return results.map((r, i) =>
  r.status === "fulfilled"
    ? { ok: true, data: r.value }
    : { ok: false, error: r.reason, retry: loaders[i] },
);

Render each card with success or per-card error + retry.

Pattern: atomic with cancel-on-fail

const ac = new AbortController();
try {
  const results = await Promise.all([
    fetch("/a", { signal: ac.signal }),
    fetch("/b", { signal: ac.signal }),
    fetch("/c", { signal: ac.signal }),
  ]);
} catch (err) {
  ac.abort();   // cancel siblings still in flight; saves bandwidth
  throw err;
}

Retry transient failures

async function retry(fn, { retries = 3, baseMs = 200 } = {}) {
  let last;
  for (let i = 0; i <= retries; i++) {
    try { return await fn(); }
    catch (e) {
      last = e;
      if (!isTransient(e) || i === retries) throw e;
      await new Promise((r) => setTimeout(r, baseMs * 2 ** i + Math.random() * 100));
    }
  }
  throw last;
}

Exponential backoff with jitter; retry only on transient errors (network, 5xx, 429), not 4xx.

Distinguish error kinds

• Network error — no response (offline, CORS, DNS).
• HTTP 4xx — client problem; don't retry blindly.
• HTTP 5xx — server problem; safe to retry.
• 429 — rate limited; honor Retry-After.
• AbortError — intentional cancel; treat as not-an-error.

Surfacing partial success

UI patterns:

• Per-item status (dashboard cards with their own error state).
• Banner "3 of 5 widgets failed to load".
• Toast for transient retries with auto-dismiss.
• Hard error for atomic operations (modal blocking next step).

Don't suppress

Either handle each rejection or let it propagate to a known error boundary. Suppressing with empty catch creates ghosts.

Interview framing

"Pick Promise.all for atomic ops (any failure invalidates the result), Promise.allSettled for independent best-effort work. For atomic ops with side-effecty calls, cancel siblings on failure with AbortController. Retry transient failures (network, 5xx, 429) with exponential backoff + jitter — never 4xx. Surface partial success explicitly: per-card errors, a 'X of Y failed' banner, not a silent half-broken UI. Treat AbortError as not-an-error in your catch."