unhandledRejection

TL;DR

An unhandledRejection event is emitted whenever a Promise is rejected and no error handler (.catch() or try...catch) is attached to it within the same turn of the event loop.

Mental Model

How It Works

Historically, Node.js would only print a deprecation warning for unhandled rejections. As of Node 15+, the default behavior is to crash the application (throw an uncaughtException), ensuring developers do not silently swallow async errors.

Example

const server = app.listen(3000);

process.on('unhandledRejection', (err) => {
    console.error('UNHANDLED REJECTION! Shutting down...');
    console.error(err.name, err.message);
    
    // Graceful shutdown: Close server, then exit
    server.close(() => {
        process.exit(1);
    });
});

// This will trigger the event because there is no .catch()
Promise.reject(new Error('Database connection failed!'));

Common Interview Questions

How does unhandledRejection differ from uncaughtException?

• uncaughtException: Triggered by synchronous errors. The process is in a critical, unknown state and must be exited synchronously and immediately.
• unhandledRejection: Triggered by asynchronous Promises. Because it is asynchronous, you usually have time to perform a graceful shutdown (finish pending HTTP requests, close DB connections) before calling process.exit(1).

How can you prevent unhandledRejection in Express routes?

By making sure every async route handler is wrapped in a try...catch block, or by using a package like express-async-errors which automatically passes rejected promises to your Express error-handling middleware.