# Writing Custom Middleware — Express.js

Source: https://www.geekswithgeeks.com/en/expressjs/ex-custom-middleware

> Build your own reusable request-handling steps.

## A simple gate-keeper

Check a header and either continue or stop the chain by sending a response.

```javascript
function requireApiKey(req, res, next) {
  if (req.headers['x-api-key'] !== 'secret123') {
    return res.status(401).json({ error: 'Missing or invalid API key' });
  }
  next();
}

app.get('/admin', requireApiKey, (req, res) => {
  res.send('Welcome, admin');
});
```

## Global vs per-route

`app.use(fn)` applies to every request. Passing `fn` as an extra argument to a route, like `app.get('/admin', fn, handler)`, applies it only there.

**Quiz:** What happens if a middleware never calls next() or sends a response?

- [ ] Express skips to the next route automatically
- [x] The request hangs — the client waits forever
- [ ] Node throws a compile error

*Answer:* The request hangs — the client waits forever. Without next() or a response, the request-response cycle never completes and the client times out.