Express.js & REST APIs

Node.js Express Error Handling

Express error-handling middleware is identified by having four parameters. Registered last, it catches errors passed to next() and, in Express 5, errors thrown from async handlers too.

What is Express Error Handling in Node.js?

Express error-handling middleware is identified by having four parameters. Registered last, it catches errors passed to next() and, in Express 5, errors thrown from async handlers too.

Express Error Handling example

Centralised error handling

JavaScript

class AppError extends Error {
  constructor(message, statusCode = 500) {
    super(message);
    this.statusCode = statusCode;
  }
}

app.get('/api/jobs/:id', async (req, res, next) => {
  const job = await Job.findById(req.params.id);
  if (!job) return next(new AppError('Job not found', 404));
  res.json(job);
});

// 404 for anything unmatched
app.use((req, res) => res.status(404).json({ error: 'Route not found' }));

// error handler — four parameters, registered LAST
app.use((err, req, res, next) => {
  const status = err.statusCode ?? 500;
  logger.error({ err, url: req.originalUrl });
  res.status(status).json({
    error: status === 500 ? 'Internal server error' : err.message,
  });
});

How this works

Generic messages are returned for 500s so stack traces and internal details never reach a client, while the real error is logged server-side.

Key points to remember

Express 4 needs a wrapper to forward async errors; Express 5 handles them natively.
Never send err.stack to the client in production.
One error handler keeps error responses consistent across the whole API.

Common mistakes with Express Error Handling

A three-parameter function used as an error handler — Express treats it as normal middleware.
Registering the error handler before the routes.