Node.js process crashes with an unhandled promise rejection instead of just warning

What causes this error

Older Node versions just logged a DeprecationWarning for unhandled rejections and kept running. Since Node 15, the default behavior (--unhandled-rejections=throw) is to raise the rejection as an uncaught exception, which crashes the process — treating it with the same severity as a synchronous uncaught throw.

The Fix

// Add a safety-net listener so you at least see what's failing
process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled rejection:', reason);
  // Log it, alert on it — but the real fix is below
});

// The actual fix: every async call site needs error handling
async function loadUser(id) {
  try {
    return await db.user.findUnique({ where: { id } });
  } catch (err) {
    logger.error(err);
    throw err; // rethrow deliberately, don't swallow silently
  }
}

Common causes / variations

• A `.then()` chain missing a trailing `.catch()`
• Firing off an async function without awaiting it or attaching a handler (`doThing()` instead of `await doThing()` or `doThing().catch(...)`)
• An event handler or callback that's `async` but whose caller has no way to see the rejection

Related errors

See also: UnhandledPromiseRejectionWarning in Express route handlers.