NodeJS

Event Loop

• fs.readFileSync blocks entire server — use fs.promises.readFile or callback version
• CPU-intensive code blocks — offload to worker threads or child process
• setImmediate vs process.nextTick — nextTick runs before I/O, setImmediate after
• Long-running loops starve I/O — break into chunks with setImmediate

Async Error Handling

• Unhandled promise rejection crashes in Node 15+ — always .catch() or try/catch with await
• process.on('unhandledRejection') for global handler — log and exit gracefully
• Errors in callbacks need explicit handling — won't propagate to outer try/catch
• Promise.all fails fast — one rejection rejects all, use Promise.allSettled if need all results

CommonJS vs ESM

• "type": "module" in package.json for ESM — otherwise .mjs extension
• ESM: import x from 'y' — CommonJS: const x = require('y')
• No __dirname in ESM — use import.meta.url with fileURLToPath
• Can't require() ESM modules — use dynamic import() which returns Promise
• exports is reference to module.exports — reassigning exports = x breaks it

Environment Variables

• process.env values are always strings — PORT=3000 is "3000" not 3000
• Missing env var is undefined, not error — check explicitly or use defaults
• .env files need dotenv — not built-in, call dotenv.config() early
• Don't commit .env — use .env.example with dummy values

Memory Leaks

• Event listeners accumulate — removeListener when done, or use once
• Closures capturing large objects — nullify references when done
• Global caches grow unbounded — use LRU cache with size limit
• --max-old-space-size to increase heap — but fix leaks first

Streams

• Backpressure: write() returns false when buffer full — wait for drain event
• .pipe() handles backpressure automatically — prefer over manual read/write
• Error handling on all streams — stream.on('error', handler) or pipeline errors silently
• pipeline() over .pipe() — handles errors and cleanup properly

Common Mistakes

• JSON.parse throws on invalid JSON — wrap in try/catch
• require() is cached — same object returned on repeated calls
• Circular dependencies partially work — but export may be incomplete at require time
• async function always returns Promise — even if you return plain value
• Buffer.from(string) encoding matters — default is UTF-8, specify if different