Top Express.js Mistakes and How to Fix Them

Express.js is a popular framework for building web applications in Node.js, but even seasoned developers encounter errors that can be tricky to debug. This guide will cover some of the most common Express.js errors, explain why they occur, and provide practical fixes to get your application back on track.

1. Error: Cannot Set Headers After They Are Sent

This error usually occurs when you attempt to send multiple responses for the same request. For example, you might accidentally call res.send() or res.json() more than once in a route handler.

Example:

app.get('/example', (req, res) => {
    res.send('First response');
    res.send('Second response'); // This will throw the error.
});

Fix:
Ensure you only send one response per request. Use conditional logic or return statements to prevent further execution after sending a response.

app.get('/example', (req, res) => {
    if (!req.query.param) {
        return res.status(400).send('Bad Request');
    }
    res.send('Valid Request');
});

2. Error: Middleware Not Executing

This happens when middleware is not properly linked or next() is not called within it. Middleware functions must explicitly pass control to the next middleware or route handler.

Example:

app.use((req, res) => {
    console.log('Middleware executed');
    // Forgot to call next()
});
app.get('/test', (req, res) => {
    res.send('Hello, World!');
});

Fix:
Call next() unless the middleware ends the response.

app.use((req, res, next) => {
    console.log('Middleware executed');
    next(); // Pass control to the next middleware or route
});

3. Error: req.body Undefined

If req.body is undefined, it's likely because you forgot to use a body-parsing middleware, such as express.json() or express.urlencoded().

Example:

app.post('/submit', (req, res) => {
    console.log(req.body); // undefined
});

Fix:

Include the body-parsing middleware in your app initialization.

app.use(express.json());
app.use(express.urlencoded({ extended: true }));

app.post('/submit', (req, res) => {
    console.log(req.body); // Now it works
    res.send('Data received');
});

4. Error: Route Not Found (404)

This error occurs when no route matches the incoming request. By default, Express doesn’t provide a 404 handler.

Fix:

Add a catch-all middleware at the end of your route definitions to handle 404 errors.

app.use((req, res) => {
    res.status(404).send('Page Not Found');
});

5. Error: EADDRINUSE (Port in Use)

This happens when another process is already using the port your app is trying to bind to.

Error: listen EADDRINUSE: address already in use :::3000

Fix:
Find and terminate the conflicting process or use a different port. You can also handle the error programmatically:

const port = process.env.PORT || 3000;

app.listen(port, () => {
    console.log(`Server running on port ${port}`);
}).on('error', (err) => {
    if (err.code === 'EADDRINUSE') {
        console.error(`Port ${port} is already in use. Please use a different port.`);
    }
});

Express.js errors can be frustrating, but understanding their root causes makes them easier to solve. With these common fixes, you’ll be better equipped to debug your applications and keep your projects running smoothly.

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