What is Node.js and what makes it different from a browser runtime?

Node.js is not a language or a framework. It is a runtime that embeds V8 to execute JavaScript and adds platform APIs for files, sockets, processes and cryptography. libuv provides the event loop, thread pool and cross-platform I/O primitives underneath those APIs.

What it gives you

• Server-side JavaScript with a shared language across the stack
• Non-blocking I/O that handles many concurrent connections with few threads
• A large package ecosystem through npm
• Cross-platform access to the OS, streams, buffers and child processes
• One process model that can be scaled with clusters, workers or multiple machines

import { createServer } from "node:http";

const server = createServer((req, res) => {
  res.writeHead(200, { "content-type": "application/json" });
  res.end(JSON.stringify({ ok: true, path: req.url }));
});

server.listen(3000);

The browser runtime is sandboxed around a document and a user. Node has no DOM and no window, but it can open sockets, read files and spawn processes. That power is why Node is common for APIs, tooling, real-time services and background jobs.