# Node.js Express Patterns

> Structure Express applications with middleware chains, routers, and proper error handling

## When to Use

- Building REST APIs with Express
- Structuring middleware for authentication, logging, and error handling
- Organizing routes with Express Router
- Adding proper error handling to async route handlers

## Instructions

1. **Application setup:**

```typescript
import express from 'express';

const app = express();

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

2. **Router organization:**

```typescript
// routes/users.ts
import { Router } from 'express';

const router = Router();

router.get('/', async (req, res) => {
  const users = await userService.findAll();
  res.json(users);
});

router.get('/:id', async (req, res) => {
  const user = await userService.findById(req.params.id);
  if (!user) return res.status(404).json({ error: 'User not found' });
  res.json(user);
});

router.post('/', async (req, res) => {
  const user = await userService.create(req.body);
  res.status(201).json(user);
});

export { router as userRouter };

// app.ts
app.use('/api/users', userRouter);
```

3. **Async error wrapper** — Express does not catch async errors by default:

```typescript
type AsyncHandler = (req: Request, res: Response, next: NextFunction) => Promise<void>;

function asyncHandler(fn: AsyncHandler) {
  return (req: Request, res: Response, next: NextFunction) => {
    fn(req, res, next).catch(next);
  };
}

router.get(
  '/:id',
  asyncHandler(async (req, res) => {
    const user = await userService.findById(req.params.id);
    if (!user) throw new NotFoundError('User', req.params.id);
    res.json(user);
  })
);
```

4. **Middleware order matters:**

```typescript
// 1. Parsing (runs first)
app.use(express.json());

// 2. Logging
app.use(requestLogger);

// 3. Authentication
app.use('/api', authenticate);

// 4. Routes
app.use('/api/users', userRouter);
app.use('/api/posts', postRouter);

// 5. 404 handler (after all routes)
app.use((req, res) => res.status(404).json({ error: 'Not found' }));

// 6. Error handler (must have 4 parameters)
app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  console.error(err);
  res.status(500).json({ error: 'Internal server error' });
});
```

5. **Request validation middleware:**

```typescript
import { z, ZodSchema } from 'zod';

function validate(schema: ZodSchema) {
  return (req: Request, res: Response, next: NextFunction) => {
    const result = schema.safeParse(req.body);
    if (!result.success) {
      return res.status(400).json({ errors: result.error.flatten().fieldErrors });
    }
    req.body = result.data;
    next();
  };
}

const CreateUserSchema = z.object({
  name: z.string().min(1),
  email: z.string().email(),
});

router.post(
  '/',
  validate(CreateUserSchema),
  asyncHandler(async (req, res) => {
    const user = await userService.create(req.body); // req.body is validated
    res.status(201).json(user);
  })
);
```

6. **Authentication middleware:**

```typescript
async function authenticate(req: Request, res: Response, next: NextFunction) {
  const token = req.headers.authorization?.replace('Bearer ', '');
  if (!token) return res.status(401).json({ error: 'No token provided' });

  try {
    const payload = await verifyToken(token);
    req.userId = payload.userId;
    next();
  } catch {
    res.status(401).json({ error: 'Invalid token' });
  }
}
```

7. **Graceful shutdown:**

```typescript
const server = app.listen(3000);

process.on('SIGTERM', () => {
  server.close(() => process.exit(0));
});
```

## Details

Express uses a middleware chain pattern where each middleware receives `(req, res, next)` and either responds or calls `next()` to pass control to the next middleware.

**Error handling middleware** must have exactly 4 parameters `(err, req, res, next)`. Express identifies error handlers by their arity. Place them after all routes.

**Express 5 (beta):** Automatically catches async errors without the `asyncHandler` wrapper. Until Express 5 is stable, the wrapper is required.

**Trade-offs:**

- Express is the most popular Node.js framework — but lacks built-in TypeScript types, validation, and async error handling
- Middleware chains are flexible — but ordering bugs are hard to diagnose
- Router modularity enables clean organization — but deeply nested routers can be confusing
- Express is mature and well-documented — but Fastify offers better performance and built-in schema validation

## Source

https://expressjs.com/en/guide/routing.html

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.
