What is the middleware pattern and how is it used in Express?

Question

Accepted Answer

**Middleware pattern** — chain of functions that process requests sequentially.

**How Express middleware works:**
```javascript
// Each middleware has (req, res, next)
app.use((req, res, next) => {
    req.requestTime = Date.now();
    next(); // Pass to next middleware
});

// Route-specific middleware
app.get("/admin", authMiddleware, adminController);

// Multiple middleware
app.post("/register",
    validateInput,
    checkDuplicates,
    createUser
);
```

**Error middleware (4 params):**
```javascript
app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).json({ error: err.message });
});
```

**Key concepts:**
- Middleware runs in order of registration.
- Must call next() or end the response.
- Errors skip to error middleware.
- app.use() for all routes, app.METHOD() for specific.

Node.js: What is the middleware pattern and how is it used in Express?