Node.js Cheat Sheet

This Node.js cheat sheet collects the code you'll reach for most often, in the same order as the course: running Node, modules, npm, files and paths, events, environment variables, async patterns, http and Express, middleware, validation, errors, streams, databases, authentication, testing, WebSockets, workers and deployment. Each row links to the lesson that explains it with examples and common mistakes.

Keep it open while you build. When you can't remember how to read a JSON body, mount a router, run a prepared statement or handle SIGTERM, a quick look here is faster than a search. Type part of a keyword into the filter box to jump straight to it, or print the page and keep it beside your keyboard.

Node.js gotchas worth remembering

• CommonJS and ES modules don't mix freely. A file using import needs "type": "module" in package.json or a .mjs extension. In ES modules, __dirname doesn't exist; build it from import.meta.url or use import.meta.dirname in recent versions.
• Forgetting await. const data = fs.promises.readFile(file) gives you a Promise, not the file. Missing await is the most common async bug in Node.js code.
• Sync methods block everyone. readFileSync is fine in a script that runs once, but inside a web server it pauses every request until the file is read.
• Express middleware order matters. app.use(express.json()) must come before the routes that read req.body, and the error handler with four arguments must come last.
• Send one response only. Calling res.send twice, often because a return is missing after an early response, causes "Cannot set headers after they are sent".
• Unhandled rejections crash the process. In Express 4, an error thrown inside an async route isn't caught automatically. Catch it and call next(err), or use Express 5, which handles rejected promises for you.
• Environment variables are strings. process.env.PORT is "3000", not 3000, and a missing variable is undefined. Convert and validate them when the app starts.
• Never commit .env files. Add .env to .gitignore and keep an .env.example with the variable names but no real secrets.

Which sections to learn first

You don't need every line of this sheet to build something useful. Start with modules, npm scripts, reading and writing files with fs/promises, and an Express app with a few routes and express.json(). That's enough to build a small REST API. Streams, worker threads, WebSockets and deployment are powerful, but you'll reach for them once an app needs them, so treat those sections as a reference to return to rather than something to learn by heart on day one.

A quick revision routine

Pick one section a day and type each snippet into a file and run it with node. Seeing real output fixes the syntax in your memory far better than reading, and the Node.js exercises give you more practice on any section that still feels shaky.