Node.js

A runtime that lets JavaScript run outside the browser — on a server, on a build machine, in a CLI tool. Built on the V8 JavaScript engine from Chrome.

When you'd see it: Most modern backend services that aren't in Python, Go, or Java. Most frontend build tooling (webpack, vite, esbuild) runs on Node. CLIs like Wrangler, Vercel, Netlify, and most LLM SDKs ship as Node packages.

Why it matters: Node turned JavaScript from a browser-only language into a general-purpose one. You can write the same language on both sides of the wire, which simplifies tooling, hiring, and code sharing. It's also the de-facto build-machine runtime for the web.

Common mistakes: Treating Node as interchangeable with browser JavaScript. Node has file-system access, environment variables, and process control that browsers don't. Browser JavaScript has the DOM and Web APIs that Node doesn't. Code written for one usually doesn't run on the other without adaptation.

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