No unhandled promise rejections in production

Unhandled promise rejections crash Node.js server processes outright in newer runtimes, and silently swallow errors in the browser — making failures invisible to both users and monitoring. CWE-391 (Unchecked Error Condition) and CWE-755 (Improper Handling of Exceptional Conditions) both apply: async errors in event handlers and useEffect callbacks that lack try/catch or .catch() chains produce no user feedback and no diagnostic signal. ISO 25010 reliability.fault-tolerance requires that faults be contained — unguarded async paths let errors propagate with no containment boundary. This is especially acute in authentication and payment flows, where a silently failed async call leaves users in an ambiguous state.

Add a global safety net in your app entry point for any rejections that escape per-call handling, then add per-call try/catch in all async event handlers.

// In _app.tsx, main.tsx, or root layout.tsx (client component)
if (typeof window !== 'undefined') {
  window.addEventListener('unhandledrejection', (event) => {
    console.error('Unhandled rejection:', event.reason)
    // Report to your error monitoring service here
  })
}

For each async event handler, wrap the body:

async function handleSubmit() {
  try {
    await submitForm(data)
  } catch (error) {
    setErrorMessage('Submission failed. Please try again.')
    reportError(error)
  }
}

Verify by triggering a deliberate rejection in a development click handler and confirming it is caught rather than surfacing in the browser console unhandled.