aranroig.com/.agents/skills/nodejs-backend-patterns/SKILL.md

---
name: nodejs-backend-patterns
description: Build production-ready Node.js backend services with Express/Fastify, implementing middleware patterns, error handling, authentication, database integration, and API design best practices. Use when creating Node.js servers, REST APIs, GraphQL backends, or microservices architectures.
---

# Node.js Backend Patterns

Comprehensive guidance for building scalable, maintainable, and production-ready Node.js backend applications with modern frameworks, architectural patterns, and best practices.

## When to Use This Skill

- Building REST APIs or GraphQL servers
- Creating microservices with Node.js
- Implementing authentication and authorization
- Designing scalable backend architectures
- Setting up middleware and error handling
- Integrating databases (SQL and NoSQL)
- Building real-time applications with WebSockets
- Implementing background job processing

## Core Frameworks

### Express.js - Minimalist Framework

**Basic Setup:**

```typescript
import express, { Request, Response, NextFunction } from "express";
import helmet from "helmet";
import cors from "cors";
import compression from "compression";

const app = express();

// Security middleware
app.use(helmet());
app.use(cors({ origin: process.env.ALLOWED_ORIGINS?.split(",") }));
app.use(compression());

// Body parsing
app.use(express.json({ limit: "10mb" }));
app.use(express.urlencoded({ extended: true, limit: "10mb" }));

// Request logging
app.use((req: Request, res: Response, next: NextFunction) => {
  console.log(`${req.method} ${req.path}`);
  next();
});

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});
```

### Fastify - High Performance Framework

**Basic Setup:**

```typescript
import Fastify from "fastify";
import helmet from "@fastify/helmet";
import cors from "@fastify/cors";
import compress from "@fastify/compress";

const fastify = Fastify({
  logger: {
    level: process.env.LOG_LEVEL || "info",
    transport: {
      target: "pino-pretty",
      options: { colorize: true },
    },
  },
});

// Plugins
await fastify.register(helmet);
await fastify.register(cors, { origin: true });
await fastify.register(compress);

// Type-safe routes with schema validation
fastify.post<{
  Body: { name: string; email: string };
  Reply: { id: string; name: string };
}>(
  "/users",
  {
    schema: {
      body: {
        type: "object",
        required: ["name", "email"],
        properties: {
          name: { type: "string", minLength: 1 },
          email: { type: "string", format: "email" },
        },
      },
    },
  },
  async (request, reply) => {
    const { name, email } = request.body;
    return { id: "123", name };
  },
);

await fastify.listen({ port: 3000, host: "0.0.0.0" });
```

## Architectural Patterns

### Pattern 1: Layered Architecture

**Structure:**

```
src/
├── controllers/     # Handle HTTP requests/responses
├── services/        # Business logic
├── repositories/    # Data access layer
├── models/          # Data models
├── middleware/      # Express/Fastify middleware
├── routes/          # Route definitions
├── utils/           # Helper functions
├── config/          # Configuration
└── types/           # TypeScript types
```

**Controller Layer:**

```typescript
// controllers/user.controller.ts
import { Request, Response, NextFunction } from "express";
import { UserService } from "../services/user.service";
import { CreateUserDTO, UpdateUserDTO } from "../types/user.types";

export class UserController {
  constructor(private userService: UserService) {}

  async createUser(req: Request, res: Response, next: NextFunction) {
    try {
      const userData: CreateUserDTO = req.body;
      const user = await this.userService.createUser(userData);
      res.status(201).json(user);
    } catch (error) {
      next(error);
    }
  }

  async getUser(req: Request, res: Response, next: NextFunction) {
    try {
      const { id } = req.params;
      const user = await this.userService.getUserById(id);
      res.json(user);
    } catch (error) {
      next(error);
    }
  }

  async updateUser(req: Request, res: Response, next: NextFunction) {
    try {
      const { id } = req.params;
      const updates: UpdateUserDTO = req.body;
      const user = await this.userService.updateUser(id, updates);
      res.json(user);
    } catch (error) {
      next(error);
    }
  }

  async deleteUser(req: Request, res: Response, next: NextFunction) {
    try {
      const { id } = req.params;
      await this.userService.deleteUser(id);
      res.status(204).send();
    } catch (error) {
      next(error);
    }
  }
}
```

**Service Layer:**

```typescript
// services/user.service.ts
import { UserRepository } from "../repositories/user.repository";
import { CreateUserDTO, UpdateUserDTO, User } from "../types/user.types";
import { NotFoundError, ValidationError } from "../utils/errors";
import bcrypt from "bcrypt";

export class UserService {
  constructor(private userRepository: UserRepository) {}

  async createUser(userData: CreateUserDTO): Promise<User> {
    // Validation
    const existingUser = await this.userRepository.findByEmail(userData.email);
    if (existingUser) {
      throw new ValidationError("Email already exists");
    }

    // Hash password
    const hashedPassword = await bcrypt.hash(userData.password, 10);

    // Create user
    const user = await this.userRepository.create({
      ...userData,
      password: hashedPassword,
    });

    // Remove password from response
    const { password, ...userWithoutPassword } = user;
    return userWithoutPassword as User;
  }

  async getUserById(id: string): Promise<User> {
    const user = await this.userRepository.findById(id);
    if (!user) {
      throw new NotFoundError("User not found");
    }
    const { password, ...userWithoutPassword } = user;
    return userWithoutPassword as User;
  }

  async updateUser(id: string, updates: UpdateUserDTO): Promise<User> {
    const user = await this.userRepository.update(id, updates);
    if (!user) {
      throw new NotFoundError("User not found");
    }
    const { password, ...userWithoutPassword } = user;
    return userWithoutPassword as User;
  }

  async deleteUser(id: string): Promise<void> {
    const deleted = await this.userRepository.delete(id);
    if (!deleted) {
      throw new NotFoundError("User not found");
    }
  }
}
```

**Repository Layer:**

```typescript
// repositories/user.repository.ts
import { Pool } from "pg";
import { CreateUserDTO, UpdateUserDTO, UserEntity } from "../types/user.types";

export class UserRepository {
  constructor(private db: Pool) {}

  async create(
    userData: CreateUserDTO & { password: string },
  ): Promise<UserEntity> {
    const query = `
      INSERT INTO users (name, email, password)
      VALUES ($1, $2, $3)
      RETURNING id, name, email, password, created_at, updated_at
    `;
    const { rows } = await this.db.query(query, [
      userData.name,
      userData.email,
      userData.password,
    ]);
    return rows[0];
  }

  async findById(id: string): Promise<UserEntity | null> {
    const query = "SELECT * FROM users WHERE id = $1";
    const { rows } = await this.db.query(query, [id]);
    return rows[0] || null;
  }

  async findByEmail(email: string): Promise<UserEntity | null> {
    const query = "SELECT * FROM users WHERE email = $1";
    const { rows } = await this.db.query(query, [email]);
    return rows[0] || null;
  }

  async update(id: string, updates: UpdateUserDTO): Promise<UserEntity | null> {
    const fields = Object.keys(updates);
    const values = Object.values(updates);

    const setClause = fields
      .map((field, idx) => `${field} = $${idx + 2}`)
      .join(", ");

    const query = `
      UPDATE users
      SET ${setClause}, updated_at = CURRENT_TIMESTAMP
      WHERE id = $1
      RETURNING *
    `;

    const { rows } = await this.db.query(query, [id, ...values]);
    return rows[0] || null;
  }

  async delete(id: string): Promise<boolean> {
    const query = "DELETE FROM users WHERE id = $1";
    const { rowCount } = await this.db.query(query, [id]);
    return rowCount > 0;
  }
}
```

### Pattern 2: Dependency Injection

Use a DI container to wire up repositories, services, and controllers. For a full container implementation, see [references/advanced-patterns.md](references/advanced-patterns.md).

## Middleware Patterns

### Authentication Middleware

```typescript
// middleware/auth.middleware.ts
import { Request, Response, NextFunction } from "express";
import jwt from "jsonwebtoken";
import { UnauthorizedError } from "../utils/errors";

interface JWTPayload {
  userId: string;
  email: string;
}

declare global {
  namespace Express {
    interface Request {
      user?: JWTPayload;
    }
  }
}

export const authenticate = async (
  req: Request,
  res: Response,
  next: NextFunction,
) => {
  try {
    const token = req.headers.authorization?.replace("Bearer ", "");

    if (!token) {
      throw new UnauthorizedError("No token provided");
    }

    const payload = jwt.verify(token, process.env.JWT_SECRET!) as JWTPayload;

    req.user = payload;
    next();
  } catch (error) {
    next(new UnauthorizedError("Invalid token"));
  }
};

export const authorize = (...roles: string[]) => {
  return async (req: Request, res: Response, next: NextFunction) => {
    if (!req.user) {
      return next(new UnauthorizedError("Not authenticated"));
    }

    // Check if user has required role
    const hasRole = roles.some((role) => req.user?.roles?.includes(role));

    if (!hasRole) {
      return next(new UnauthorizedError("Insufficient permissions"));
    }

    next();
  };
};
```

### Validation Middleware

```typescript
// middleware/validation.middleware.ts
import { Request, Response, NextFunction } from "express";
import { AnyZodObject, ZodError } from "zod";
import { ValidationError } from "../utils/errors";

export const validate = (schema: AnyZodObject) => {
  return async (req: Request, res: Response, next: NextFunction) => {
    try {
      await schema.parseAsync({
        body: req.body,
        query: req.query,
        params: req.params,
      });
      next();
    } catch (error) {
      if (error instanceof ZodError) {
        const errors = error.errors.map((err) => ({
          field: err.path.join("."),
          message: err.message,
        }));
        next(new ValidationError("Validation failed", errors));
      } else {
        next(error);
      }
    }
  };
};

// Usage with Zod
import { z } from "zod";

const createUserSchema = z.object({
  body: z.object({
    name: z.string().min(1),
    email: z.string().email(),
    password: z.string().min(8),
  }),
});

router.post("/users", validate(createUserSchema), userController.createUser);
```

### Rate Limiting Middleware

```typescript
// middleware/rate-limit.middleware.ts
import rateLimit from "express-rate-limit";
import RedisStore from "rate-limit-redis";
import Redis from "ioredis";

const redis = new Redis({
  host: process.env.REDIS_HOST,
  port: parseInt(process.env.REDIS_PORT || "6379"),
});

export const apiLimiter = rateLimit({
  store: new RedisStore({
    client: redis,
    prefix: "rl:",
  }),
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100, // Limit each IP to 100 requests per windowMs
  message: "Too many requests from this IP, please try again later",
  standardHeaders: true,
  legacyHeaders: false,
});

export const authLimiter = rateLimit({
  store: new RedisStore({
    client: redis,
    prefix: "rl:auth:",
  }),
  windowMs: 15 * 60 * 1000,
  max: 5, // Stricter limit for auth endpoints
  skipSuccessfulRequests: true,
});
```

### Request Logging Middleware

```typescript
// middleware/logger.middleware.ts
import { Request, Response, NextFunction } from "express";
import pino from "pino";

const logger = pino({
  level: process.env.LOG_LEVEL || "info",
  transport: {
    target: "pino-pretty",
    options: { colorize: true },
  },
});

export const requestLogger = (
  req: Request,
  res: Response,
  next: NextFunction,
) => {
  const start = Date.now();

  // Log response when finished
  res.on("finish", () => {
    const duration = Date.now() - start;
    logger.info({
      method: req.method,
      url: req.url,
      status: res.statusCode,
      duration: `${duration}ms`,
      userAgent: req.headers["user-agent"],
      ip: req.ip,
    });
  });

  next();
};

export { logger };
```

## Error Handling

### Custom Error Classes

```typescript
// utils/errors.ts
export class AppError extends Error {
  constructor(
    public message: string,
    public statusCode: number = 500,
    public isOperational: boolean = true,
  ) {
    super(message);
    Object.setPrototypeOf(this, AppError.prototype);
    Error.captureStackTrace(this, this.constructor);
  }
}

export class ValidationError extends AppError {
  constructor(
    message: string,
    public errors?: any[],
  ) {
    super(message, 400);
  }
}

export class NotFoundError extends AppError {
  constructor(message: string = "Resource not found") {
    super(message, 404);
  }
}

export class UnauthorizedError extends AppError {
  constructor(message: string = "Unauthorized") {
    super(message, 401);
  }
}

export class ForbiddenError extends AppError {
  constructor(message: string = "Forbidden") {
    super(message, 403);
  }
}

export class ConflictError extends AppError {
  constructor(message: string) {
    super(message, 409);
  }
}
```

### Global Error Handler

```typescript
// middleware/error-handler.ts
import { Request, Response, NextFunction } from "express";
import { AppError } from "../utils/errors";
import { logger } from "./logger.middleware";

export const errorHandler = (
  err: Error,
  req: Request,
  res: Response,
  next: NextFunction,
) => {
  if (err instanceof AppError) {
    return res.status(err.statusCode).json({
      status: "error",
      message: err.message,
      ...(err instanceof ValidationError && { errors: err.errors }),
    });
  }

  // Log unexpected errors
  logger.error({
    error: err.message,
    stack: err.stack,
    url: req.url,
    method: req.method,
  });

  // Don't leak error details in production
  const message =
    process.env.NODE_ENV === "production"
      ? "Internal server error"
      : err.message;

  res.status(500).json({
    status: "error",
    message,
  });
};

// Async error wrapper
export const asyncHandler = (
  fn: (req: Request, res: Response, next: NextFunction) => Promise<any>,
) => {
  return (req: Request, res: Response, next: NextFunction) => {
    Promise.resolve(fn(req, res, next)).catch(next);
  };
};
```

## Database Patterns

Node.js supports both SQL and NoSQL databases. Use connection pooling for all production databases.

Key patterns covered in [references/advanced-patterns.md](references/advanced-patterns.md):
- **PostgreSQL with connection pool** — `pg` Pool configuration and graceful shutdown
- **MongoDB with Mongoose** — connection management and schema definition
- **Transaction pattern** — `BEGIN`/`COMMIT`/`ROLLBACK` with `pg` client

## Authentication & Authorization

JWT-based auth with access tokens (short-lived, 15m) and refresh tokens (7d). Full `AuthService` implementation with `bcrypt` password comparison in [references/advanced-patterns.md](references/advanced-patterns.md).

## Caching Strategies

Redis-backed `CacheService` with get/set/delete/invalidatePattern, plus a `@Cacheable` decorator for method-level caching. See [references/advanced-patterns.md](references/advanced-patterns.md).

## API Response Format

Standardized `ApiResponse` helper with `success`, `error`, and `paginated` static methods. See [references/advanced-patterns.md](references/advanced-patterns.md).

## Best Practices

1. **Use TypeScript**: Type safety prevents runtime errors
2. **Implement proper error handling**: Use custom error classes
3. **Validate input**: Use libraries like Zod or Joi
4. **Use environment variables**: Never hardcode secrets
5. **Implement logging**: Use structured logging (Pino, Winston)
6. **Add rate limiting**: Prevent abuse
7. **Use HTTPS**: Always in production
8. **Implement CORS properly**: Don't use `*` in production
9. **Use dependency injection**: Easier testing and maintenance
10. **Write tests**: Unit, integration, and E2E tests
11. **Handle graceful shutdown**: Clean up resources
12. **Use connection pooling**: For databases
13. **Implement health checks**: For monitoring
14. **Use compression**: Reduce response size
15. **Monitor performance**: Use APM tools

## Testing Patterns

See `javascript-testing-patterns` skill for comprehensive testing guidance.