What is the difference between uncaughtException and unhandledRejection?

Direct answer

uncaughtException fires on synchronous throws with no try/catch; unhandledRejection fires when a Promise rejects with no .catch — Node may exit on uncaughtException; treat both as bugs in production, log, graceful shutdown, and use --unhandled-rejections=strict in CI.

These process-level events catch errors that escaped application boundaries — last-resort hooks, not substitutes for handler try/catch.

Event	Trigger	Typical cause
uncaughtException	Sync throw outside try/catch	JSON.parse in top-level, bug in sync middleware
unhandledRejection	Promise rejected, no handler	Missing await, forgotten .catch on async call
warning (deprecated)	Some versions warn before exit	Floating promise in background task

Do not resume normally — after uncaughtException, process state may be corrupt; shutdown and let PM2/Docker restart.
Graceful shutdown — close server, drain connections, then process.exit(1).
Framework safety — Fastify catches async route rejections; manual setImmediate async still needs .catch.

process-errors.mjs

process.on("uncaughtException", (err) => {
  console.error("uncaughtException:", err.stack);
  shutdown(1);
});

process.on("unhandledRejection", (reason) => {
  console.error("unhandledRejection:", reason);
  shutdown(1);
});

function shutdown(code) {
  server.close(() => process.exit(code));
  setTimeout(() => process.exit(code), 10_000).unref();
}