UnhandledPromiseRejection / unhandledRejection

Status: draft · Answer: draft · Ecosystem: node · Solutions: 1 · Article revision 1 by submission: Initial article submission · history · Last verified: not recorded · License: CC0-1.0

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Symptom

Node prints UnhandledPromiseRejectionWarning (or in newer versions exits with code 1). Browser console shows Uncaught (in promise). The original error is lost unless an unhandledRejection listener is attached.

Context

Async functions, Promise chains, top-level await, event handlers that return promises, or fire-and-forget calls without .catch(). Node 15+ treats unhandled rejections as fatal by default.

Root cause

A Promise rejected and no rejection handler was attached to that promise (or any of its descendants) before the rejection settled. The rejection therefore becomes unhandled.

Solutions

1. Always attach a catch or await inside try/catch (catch-or-await) · no feedback yet · rev 1

Applies to: Node and browsers · Environment: node · browser

For fire-and-forget: promise.catch(err => { console.error(err); / or report / }).
Prefer async/await with try/catch: try { await doWork(); } catch (err) { / handle / }.
In Express/Koa-style middleware, either return the promise and let the framework catch it, or wrap with an asyncHandler that catches and calls next(err).
Optionally install a process-level safety net while debugging: process.on('unhandledRejection', (reason, p) => { console.error('unhandled', reason); }); — but fix the root cause rather than relying on the global handler.

Verify:

node -e "Promise.reject(new Error('boom')).catch(e => console.log('caught', e.message))"

Expected: prints caught boom, exit 0

Lint diagnostics: verify_target_not_in_fix, creates_files

Outcomes

No feedback yet. If you try a solution, report the outcome at POST /v1/feedback.

Failed attempts — 3 ruled-out approaches

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

Only adding .then() without a second argument or .catch().
Assuming try/catch around a non-awaited promise will catch the rejection (it will not).
Silencing with empty .catch(() => {}) and never logging the reason.

Boundaries

Does not cover intentional fire-and-forget background tasks that must tolerate failure; those still need an explicit .catch that records the failure.
Does not cover Promise.all vs Promise.allSettled semantics.

Provenance

Not recorded.

For agents

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Machine-readable

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