How to Handle Errors in an Express Server

How to Handle Errors in an Express Server

Error handling is what separates a learning Express server from a production one. Here is the proper way.

1. Use an Error-Handling Middleware

Add this at the end of app.js with four parameters:

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(err.status || 500).json({
    error: err.message || 'Something went wrong'
  });
});

Express detects the four parameters and uses this middleware for errors. Register it after all routes.

2. Throw or Pass Errors With next(err)

In a handler, throw an error or call next(err). Express jumps straight to the error middleware. Example:

if (!user) return next(new Error('User not found'));

3. Use a Custom Error Class

Subclass Error to add a status and a safe message:

class ApiError extends Error {
  constructor(status, message) {
    super(message);
    this.status = status;
  }
}

throw new ApiError(404, 'User not found');

This keeps errors consistent and prevents leaking internals.

4. Catch Async Errors

Async handlers throw promises that Express 4 does not catch. Use a wrapper:

const asyncHandler = fn => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

router.get('/users/:id', asyncHandler(async (req, res) => {
  const user = await User.findById(req.params.id);
  if (!user) throw new ApiError(404, 'User not found');
  res.json(user);
}));

In Express 5, async errors are caught automatically. The wrapper is still useful for older versions.

5. Use Status Codes Correctly

• 400: bad request (validation).
• 401: not authenticated.
• 403: not authorized.
• 404: not found.
• 409: conflict (duplicate).
• 422: unprocessable entity (semantic error).
• 500: server error.
• 503: maintenance or downstream failure.

6. Never Leak Stack Traces in Production

In development, send the full error. In production, send a safe message. Use a NODE_ENV check:

res.status(err.status || 500).json({
  error: process.env.NODE_ENV === 'production'
    ? 'Something went wrong'
    : err.message
});

7. Log Errors Externally

console.error is fine in dev. In production, use a logger (winston, pino) and ship logs to a central place. For real apps, integrate Sentry or similar to get error alerts.

The Takeaway

Handle errors in Express with an error-handling middleware, throw or next(err), a custom ApiError class, an async wrapper, correct status codes, no stack trace leaks in production, and external logging. Done right, errors become informative, not crashes.