Node.js Backend Conventions

This skill consolidates idioms that hold across most Node.js backend projects (Express, Fastify, Koa, plain Node). Follow them when adding code; if a project's CLAUDE.md contradicts a convention here, the project wins.

Project layout

Typical layout:

project-root/
├── package.json
├── tsconfig.json              # if TypeScript
├── src/
│   ├── index.js               # entry point — start server, wire app
│   ├── app.js                 # framework instance (express(), fastify())
│   ├── config.js              # env-var reader (single source)
│   ├── routes/                # route definitions, one file per resource
│   ├── controllers/           # request handlers (or merged into routes)
│   ├── services/              # business logic, framework-agnostic
│   ├── middleware/
│   ├── lib/                   # shared utilities
│   └── db/                    # data access layer
├── test/  OR  __tests__/
└── dist/                      # build output (TS), gitignored

Mirror what exists. If the project uses a flat layout, do not introduce nested layout for one new file.

Configuration

Single config module reading from process.env:

// src/config.js
const requiredEnv = ['DATABASE_URL', 'PORT'];
for (const key of requiredEnv) {
  if (!process.env[key]) throw new Error(`Missing env var: ${key}`);
}

module.exports = {
  port: Number(process.env.PORT),
  databaseUrl: process.env.DATABASE_URL,
  logLevel: process.env.LOG_LEVEL || 'info',
};

Never read process.env from business logic. Always go through config.

Logging

Use the project's existing logger. Detection priority:

1. pino in deps → const logger = require('pino')()
2. winston in deps → use the configured instance
3. bunyan in deps → use the configured instance
4. Otherwise → console.log/console.error is acceptable

Never introduce a new logger as part of feature work without justification in DECISIONS.

Log levels: debug (verbose dev), info (normal flow), warn (recoverable), error (something failed).

Routing patterns

Express

// src/routes/health.js
const router = require('express').Router();
const { getHealth } = require('../controllers/health');

router.get('/healthz', getHealth);

module.exports = router;

Mount in src/app.js:

app.use(require('./routes/health'));

Fastify

// src/routes/health.js
async function healthRoutes(fastify) {
  fastify.get('/healthz', async () => ({ status: 'ok' }));
}
module.exports = healthRoutes;

Register in src/app.js:

fastify.register(require('./routes/health'));

Error handling

Async route handlers — never let rejections escape

Express requires explicit next(err). Two patterns:

Pattern A — wrapper:

const asyncHandler = (fn) => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

router.get('/x', asyncHandler(async (req, res) => {
  const data = await mayThrow();
  res.json(data);
}));

Pattern B — express@5+ native (auto-forwards rejections to next).

Use whatever the project already uses. Don't introduce a wrapper if the project uses express@5.

Centralized error middleware

// src/middleware/error.js
module.exports = (err, req, res, next) => {
  logger.error({ err }, 'request failed');
  const status = err.status || 500;
  res.status(status).json({ error: err.message });
};

Always last in the middleware chain.

Fastify: setErrorHandler

fastify.setErrorHandler((err, req, reply) => {
  logger.error({ err }, 'request failed');
  reply.status(err.statusCode || 500).send({ error: err.message });
});

Validation

At the request boundary, not in business logic. Use what's installed:

• zod → schema.parse(req.body) inside try/catch.
• joi → schema.validate(req.body, { abortEarly: false }).
• ajv → compiled validator instance.
• Express → express-validator middleware on the route.

If nothing is installed and BA spec requires validation: add zod (smallest, TS-friendly) and document in DECISIONS.

Async patterns

• Prefer async/await over .then().catch().
• Don't mix CJS require and ESM import in one file. Detect from package.json "type" field.
• Don't use top-level await unless "type": "module" is set.
• For parallel work, Promise.all([a, b, c]). For settled-on-all, Promise.allSettled.

Graceful shutdown

For HTTP servers, handle SIGTERM and SIGINT:

const server = app.listen(port);

const shutdown = async () => {
  server.close(() => process.exit(0));
  setTimeout(() => process.exit(1), 10_000).unref();
};
process.on('SIGTERM', shutdown);
process.on('SIGINT', shutdown);

Drain in-flight requests, close DB pool, then exit.

Anti-patterns

• ❌ app.use((req, res) => res.status(500).send(err.stack)) — never expose stack traces in production responses.
• ❌ Reading process.env.X deep inside business logic.
• ❌ Mixing require and import in one file.
• ❌ Swallowing errors silently (catch(() => {})).
• ❌ Hardcoded ports / URLs / credentials.
• ❌ Module-level mutable state (use dependency injection or factory functions).
• ❌ eval, Function() constructor on user input.