nodejs

Node.js

Respect the event loop. Every blocking operation is a scalability bug.

Node.js rewards async-first, stream-oriented code. If your Node.js code fights the event loop, it's wrong.

References

• Module system — [${CLAUDE_SKILL_DIR}/references/modules.md]: ESM/CJS comparison tables, file extension rules, conditional exports patterns
• Event loop — [${CLAUDE_SKILL_DIR}/references/event-loop.md]: Phase order, execution priority, blocking operations table, worker pool
• Streams — [${CLAUDE_SKILL_DIR}/references/streams.md]: Stream types table, pipeline patterns, backpressure details
• Error handling — [${CLAUDE_SKILL_DIR}/references/errors.md]: Error categories table, global handlers, centralized error handling
• Security — [${CLAUDE_SKILL_DIR}/references/security.md]: Supply chain threats table, HTTP security headers, process hardening

Module System

• Use ESM. Set "type": "module" in package.json. Use .mjs/.cjs only when mixing module systems within one package.
• Use node: prefix for all built-in imports: import fs from 'node:fs'. Prevents package name collision attacks and is unambiguous.
• Import process explicitly. import process from 'node:process'. Never rely on the process global — explicit imports make dependencies visible.
• Define "exports" in package.json for libraries. Encapsulates internals — only paths listed in "exports" are importable by consumers.
• Imports at module top. No dynamic import() for statically-known dependencies.
• Use import.meta.dirname/import.meta.filename instead of __dirname/__filename.
• Use import.meta.resolve() instead of require.resolve() in ESM.
• Always set "type" explicitly in package.json, even in CJS packages — future-proofs the package and helps tooling.
• Use conditional exports for dual CJS/ESM packages. Order: types > import > require > default. Always include "default" as fallback.
• Place "types" first in conditional exports when publishing TypeScript declarations.
• Use # imports ("imports" in package.json) for clean internal paths without ../../../. Supports conditional resolution for platform-specific implementations.
• Keep "main" alongside "exports" only for backward compatibility with old Node.js or bundlers. Never use "main" alone for new packages.
• JSON imports in ESM require with { type: 'json' } attribute.
• Use createRequire() from node:module only when you must require() in ESM (e.g., native addons).
• CJS interop: default import from CJS always works. Named imports work if CJS uses static export patterns; otherwise destructure the default.
• require() can load synchronous ESM (no top-level await). For ESM with top-level await, use dynamic import().
• Self-referencing: a package can import its own exports by name when "exports" is defined.

File extension rules and ESM vs CJS comparison tables: see ${CLAUDE_SKILL_DIR}/references/modules.md.

Event Loop

Node.js uses a single-threaded event loop for JavaScript and a libuv worker pool for expensive I/O and CPU tasks.

Core Rules

• Never block the event loop. No sync I/O in servers (readFileSync, execSync, crypto.pbkdf2Sync, zlib.inflateSync). Sync APIs are acceptable only in CLI scripts, startup code, or build tools.
• Offload CPU-intensive work to worker_threads or child processes. For main-thread CPU work, partition into chunks with setImmediate() between iterations.
• Bound input sizes. Unbounded JSON.parse, JSON.stringify, regex, or iteration = DoS vector. A 50MB JSON string blocks the loop for ~2 seconds.
• Avoid vulnerable regex. No nested quantifiers (a+)*, no overlapping alternations (a|a)*, no backreferences with repetition. Use safe-regex2, RE2, or indexOf.
• Prefer setImmediate() over recursive process.nextTick(). nextTick starves I/O if called recursively. Use nextTick only when you must run before any I/O in the current tick (e.g., emitting events after construction before listeners attach).
• Prefer queueMicrotask() over process.nextTick() for new code — it's cross-platform and web-standard.
• Inside I/O callbacks, setImmediate always fires before setTimeout(fn, 0). Outside I/O, the order is non-deterministic — do not depend on it.
• Use AbortController for cancellable timers and operations.

Phase order, execution priority, blocking operations table, and worker pool details: see ${CLAUDE_SKILL_DIR}/references/event-loop.md.

Streams

Streams process data incrementally — use them for large files, HTTP bodies, data transformation pipelines, and proxying. Do not use streams when data is already fully in memory.

Core Rules

• Use pipeline() from node:stream/promises for stream composition. Never manual .pipe() chains — they don't propagate errors or handle cleanup.
• Respect backpressure. Check .write() return value; wait for 'drain' event before continuing. pipeline() handles this automatically.
• Prefer Readable.from() for converting iterables/async iterables to streams.
• Use async iteration (for await (const chunk of stream)) as the simplest way to consume readable streams. Backpressure is handled automatically.
• Use readline with createInterface for line-by-line file processing.
• highWaterMark defaults to 16 KiB for byte streams, 16 objects for object mode. It's a hint, not a hard limit.
• Object mode streams count objects (not bytes) against highWaterMark. Enable with { objectMode: true }.
• Destroy streams explicitly when you need to abort: stream.destroy(new Error('msg')).
• Custom Readable: prefer async generators with Readable.from() over class-based _read() implementation unless you need fine-grained control.
• Custom Transform: implement _transform(chunk, encoding, callback) and optionally _flush(callback) for end-of-stream processing.
• Custom Writable: implement _write(chunk, encoding, callback) and optionally _final(callback) for cleanup before 'finish' event.

Stream types table and .pipe() pitfalls: see ${CLAUDE_SKILL_DIR}/references/streams.md.

Error Handling

Core Rules

• Use async/await with try/catch. No callbacks for new code.
• Always return await when returning promises from try blocks — preserves full stack traces and ensures catch fires for rejections.
• Extend Error. Custom errors must extend Error, set a code property for programmatic matching (not message strings, which change), and set name.
• Use error.cause for chaining: new Error("context", { cause: originalErr }). The full chain is visible via util.inspect() and structured loggers.
• Match errors by error.code or instanceof, never by message string.
• Register global handlers. Always handle process.on('unhandledRejection') and process.on('uncaughtException'). Log, clean up, exit. Since Node.js 15+, unhandled rejections crash the process by default.
• Never resume after uncaughtException. The process state is unknown — log, cleanup, exit.
• Subscribe to 'error' events on all EventEmitters and streams. An unhandled 'error' event crashes the process. pipeline() handles stream errors automatically.
• Handle process.on('warning') for non-fatal process warnings (deprecations, memory leaks, experimental features).
• Use centralized error handlers. Don't scatter error handling across every middleware. Use a single error handler that maps error types to HTTP responses without leaking internals.
• Handle once: log OR throw, not both. catch (e) { log(e); throw e } causes duplicate logging.
• Never swallow errors in event handlers — always re-emit or log.

Error categories table and operational vs programmer error strategies: see ${CLAUDE_SKILL_DIR}/references/errors.md.

Process Lifecycle

• Graceful shutdown. Handle SIGTERM/SIGINT: stop accepting connections, wait for in-flight requests (with timeout), close DB pools, flush logs, then process.exit(0). Force-exit on timeout.
• Log to stdout/stderr. Let infrastructure (Docker, systemd) handle log routing. Use structured JSON logging (pino, winston) in production.
• Set NODE_ENV=production in production. It enables framework optimizations and disables debug output.
• Use npm ci in CI/production. Never npm install — it ignores lockfile mismatches.

Security

Input Validation

• Validate everything from outside — request bodies, query params, headers, file uploads, environment variables from untrusted sources. Use schema validation (zod, ajv, typebox).
• Limit request body size at the HTTP layer before parsing. Set per-content-type limits. Unbounded payloads exhaust memory.
• Validate Content-Length header before reading the body.
• Use streaming JSON parsers (stream-json, @streamparser/json) for very large JSON payloads.

HTTP Security

• Configure server timeouts. Defaults are too permissive. Set headersTimeout, requestTimeout, timeout, keepAliveTimeout, maxRequestsPerSocket.
• Use security headers (via helmet or equivalent): Strict-Transport-Security, X-Content-Type-Options: nosniff, X-Frame-Options: DENY, Content-Security-Policy.
• Delegate TLS/gzip to reverse proxy. Node.js should not terminate TLS or compress responses in production — let nginx/HAProxy/cloud LB handle TLS termination, compression, rate limiting, and WAF rules.

Secrets & Process Hardening

• Never hardcode secrets in source code. Use environment variables from secure vaults.
• Never commit .env files — add to .gitignore.
• Use crypto.timingSafeEqual() for secret comparison (prevents timing attacks).
• Use crypto.scrypt() or crypto.pbkdf2() (async versions) for password hashing.
• Avoid shell injection. Never use exec() with user-controlled strings. Use execFile() or spawn() with argument arrays.
• Never use eval(), new Function(), or dynamic require() with user input.
• Run as non-root in Docker — use a dedicated user.
• Limit V8 heap with --max-old-space-size to prevent memory exhaustion.

Supply chain threats table, dependency auditing, and security checklist: see ${CLAUDE_SKILL_DIR}/references/security.md.

Application

When writing Node.js code:

• Apply all conventions silently — don't narrate each rule being followed.
• If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.
• Prefer node:fs/promises over callback-based node:fs.
• Prefer node:stream/promises for pipeline operations.

When reviewing Node.js code:

• Cite the specific violation and show the fix inline.
• Don't lecture or quote the rule — state what's wrong and how to fix it.

Integration

The javascript skill governs language choices; this skill governs Node.js runtime decisions. Activate typescript alongside both when working with TypeScript.

Respect the event loop. When in doubt, make it async.