The Last Line of Defence

Sometimes errors occur in places you didn't wrap in try..catch. A global error handler catches everything else.

window.onerror = function(message, url, line, col, error) {
  console.log('Uncaught error!');
  console.log('Message:', message);
  console.log('File:', url, 'Line:', line);
  
  // Send to your error tracking service
  sendToErrorTracker({ message, url, line, stack: error?.stack });
  
  return true; // Prevents the browser from printing it to console
};

For Unhandled Promise Rejections

window.addEventListener('unhandledrejection', function(event) {
  console.error('Unhandled Promise rejection:', event.reason);
  sendToErrorTracker({ reason: event.reason });
});

Real-World Purpose

In production, these handlers send error reports to monitoring services like Sentry or Datadog, alerting developers when users encounter bugs — even bugs the developer never personally saw.