Node.js Backend Skill

Load with: base.md + typescript.md

Project Structure

project/
├── src/
│   ├── core/                   # Pure business logic
│   │   ├── types.ts            # Domain types
│   │   ├── errors.ts           # Domain errors
│   │   └── services/           # Pure functions
│   │       ├── user.ts
│   │       └── order.ts
│   ├── infra/                  # Side effects
│   │   ├── http/               # HTTP layer
│   │   │   ├── server.ts       # Server setup
│   │   │   ├── routes/         # Route handlers
│   │   │   └── middleware/     # Express middleware
│   │   ├── db/                 # Database
│   │   │   ├── client.ts       # DB connection
│   │   │   ├── repositories/   # Data access
│   │   │   └── migrations/     # Schema migrations
│   │   └── external/           # Third-party APIs
│   ├── config/                 # Configuration
│   │   └── index.ts            # Env vars, validated
│   └── index.ts                # Entry point
├── tests/
│   ├── unit/
│   └── integration/
├── package.json
└── CLAUDE.md

API Design

Route Handler Pattern

// routes/users.ts
import { Router } from 'express';
import { z } from 'zod';
import { createUser } from '../../core/services/user';
import { UserRepository } from '../db/repositories/user';

const CreateUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1).max(100),
});

export function createUserRoutes(userRepo: UserRepository): Router {
  const router = Router();

  router.post('/', async (req, res, next) => {
    try {
      const input = CreateUserSchema.parse(req.body);
      const user = await createUser(input, userRepo);
      res.status(201).json(user);
    } catch (error) {
      next(error);
    }
  });

  return router;
}

Dependency Injection at Composition Root

// index.ts
import { createApp } from './infra/http/server';
import { createDbClient } from './infra/db/client';
import { UserRepository } from './infra/db/repositories/user';
import { createUserRoutes } from './infra/http/routes/users';

async function main(): Promise<void> {
  const db = await createDbClient();
  const userRepo = new UserRepository(db);
  
  const app = createApp({
    userRoutes: createUserRoutes(userRepo),
  });
  
  app.listen(3000);
}

Error Handling

Domain Errors

// core/errors.ts
export class DomainError extends Error {
  constructor(
    message: string,
    public readonly code: string,
    public readonly statusCode: number = 400
  ) {
    super(message);
    this.name = 'DomainError';
  }
}

export class NotFoundError extends DomainError {
  constructor(resource: string, id: string) {
    super(`${resource} with id ${id} not found`, 'NOT_FOUND', 404);
  }
}

export class ValidationError extends DomainError {
  constructor(message: string) {
    super(message, 'VALIDATION_ERROR', 400);
  }
}

Global Error Handler

// middleware/errorHandler.ts
import { ErrorRequestHandler } from 'express';
import { DomainError } from '../../core/errors';
import { ZodError } from 'zod';

export const errorHandler: ErrorRequestHandler = (err, req, res, next) => {
  if (err instanceof DomainError) {
    return res.status(err.statusCode).json({
      error: { code: err.code, message: err.message },
    });
  }

  if (err instanceof ZodError) {
    return res.status(400).json({
      error: { code: 'VALIDATION_ERROR', details: err.errors },
    });
  }

  console.error('Unexpected error:', err);
  return res.status(500).json({
    error: { code: 'INTERNAL_ERROR', message: 'Something went wrong' },
  });
};

Database Patterns

Repository Pattern

// db/repositories/user.ts
import { Kysely } from 'kysely';
import { Database, User