How to handle unhandled promise rejections in JavaScript

Short answer

Unhandled promise rejections occur when a Promise rejects without a catch handler. Always attach .catch() or use try/catch with await to handle them.

Steps

1. Attach a catch handler: javascript fetch("/api").then(r => r.json()).catch(err => console.error(err));
2. Use try/catch with await: javascript try { const data = await fetchData(); } catch (err) { console.error(err); }
3. Listen for global unhandled rejections: javascript window.addEventListener("unhandledrejection", event => { console.error("Unhandled:", event.reason); event.preventDefault(); });
4. Handle rejections in Promise.all: javascript Promise.all(tasks).catch(err => console.error(err));

Tips

• In Node.js, unhandled rejections may terminate the process in future versions.
• Log unhandled rejections to your error tracking service.

Common issues

• Forgetting to return a Promise inside a .then() chain leaves rejection unhandled.
• Async functions that throw without a caller catch block result in unhandled rejections.