How to Handle Errors in Express: Simple Guide with Examples

In Express, handle errors by creating an error-handling middleware with four parameters: err, req, res, next. Place this middleware after all routes to catch and respond to errors gracefully.

Why This Happens

When an error occurs in an Express route but there is no error-handling middleware, the server may crash or send incomplete responses. This happens because Express does not know how to handle the error properly without a dedicated handler.

javascript

import express from 'express';
const app = express();

app.get('/', (req, res) => {
  throw new Error('Something went wrong!');
});

app.listen(3000, () => console.log('Server running on port 3000'));

Output

Error: Something went wrong!
    at ... (stack trace)
Server crashes or hangs without proper error response.

The Fix

Add an error-handling middleware function with four parameters: err, req, res, next. This middleware catches errors thrown in routes and sends a proper response without crashing the server.

javascript

import express from 'express';
const app = express();

app.get('/', (req, res) => {
  throw new Error('Something went wrong!');
});

// Error-handling middleware
app.use((err, req, res, next) => {
  console.error(err.message);
  res.status(500).send('Internal Server Error: ' + err.message);
});

app.listen(3000, () => console.log('Server running on port 3000'));

Output

When visiting '/', the server responds with status 500 and message: 'Internal Server Error: Something went wrong!' without crashing.

Prevention

Always include an error-handling middleware at the end of your middleware stack to catch errors. Use try/catch blocks or async/await with next(err) to forward errors. Use logging to track errors and avoid exposing sensitive info to users.

Linting tools like ESLint can help enforce error handling patterns.

Related Errors

Common related errors include unhandled promise rejections and missing next(err) calls in async routes. To fix, always call next(err) inside catch blocks or use middleware like express-async-errors to handle async errors automatically.

Key Takeaways

Use error-handling middleware with four parameters to catch errors in Express.

Place error middleware after all routes to handle errors globally.

Forward errors in async code using next(err) or try/catch blocks.

Log errors for debugging but avoid exposing sensitive details to users.

Use linting and middleware helpers to enforce consistent error handling.