What Is Node.js? The JavaScript Runtime Explained

Node.js is a runtime that runs JavaScript outside the browser: on a server, in a terminal, or as a command line tool. It is not a programming language (the language is JavaScript) and not a framework (Express is a framework that runs on Node.js). It combines Google's V8 engine, which executes JavaScript, with an event loop and built-in APIs for files, networking and processes that browsers do not have.

const fs = require('fs');

// Node.js can write and read files on disk
fs.writeFileSync('hello.txt', 'Written by Node.js');
console.log(fs.readFileSync('hello.txt', 'utf8'));

console.log('Running on Node', process.version.split('.')[0]);
console.log('Is there a window object?', typeof window);

It printed the file's contents, the major version (v24), and undefined for window, because Node.js has no web page to give you one. fs and process exist only in Node.js; run the same lines in a browser tab and the first one throws.

Is Node.js a framework or a language?

Neither. JavaScript is the language. Node.js is the program that runs it, the same way the JVM runs Java or CPython runs Python. Frameworks sit on top: Express, Fastify and NestJS are libraries you install into a Node.js project. The Node.js vs JavaScript page compares the two in detail.

Under the hood, Node.js is four parts:

• V8, the JavaScript engine from Chrome. It turns your code into machine code and runs it.
• libuv, a C library that provides the event loop and does slow work (disk, network, timers) in the background, then hands the result back to your callbacks.
• Node's own APIs: fs for files, http for servers, path, process, crypto, events and more, available with require or import and no install.
• npm, the package manager that ships with Node.js and gives you access to the largest package registry of any language.

How to run a Node.js file

Install Node.js from its website (pick the LTS version), then check it from a terminal and run a file:

node --version     # prints v24.x.x or similar
node app.js        # runs app.js and exits when it has nothing left to do
node               # with no file: an interactive prompt (the REPL)

A program ends by itself once nothing is waiting: no timer, no open server, no pending file read. That is why a script exits but a web server keeps running.

Non-blocking I/O, the core idea

Node.js does not wait for slow work. It starts a file read or a network request, keeps running the next lines, and calls your function when the result is ready.

const fs = require('fs');

fs.writeFileSync('data.txt', '42'); // setup: a file to read

fs.readFile('data.txt', 'utf8', (err, text) => {
    console.log('3. file contents:', text);
});

console.log('1. read started');
console.log('2. still running other code');

The two console.log lines after readFile ran first, and the file's contents arrived last. This is how one Node.js process serves thousands of connections: while one request waits for the database, the event loop keeps the others moving.

Node.js vs the browser

What Node.js is used for

• Web servers and APIs: REST and GraphQL backends, often with Express.
• Real-time apps: chat, live dashboards and multiplayer games over WebSockets, where many connections stay open at once.
• Command line tools: npm itself, Vite, ESLint, Prettier and TypeScript's compiler all run on Node.js.
• Scripts and automation: renaming files, calling APIs, migrating data.

The most common use is an HTTP server, and the built-in http module is enough for a first one:

// GET /hello
const http = require('http');

const server = http.createServer((req, res) => {
    res.setHeader('Content-Type', 'text/plain');
    res.end('Hello from Node.js. You asked for ' + req.url);
});

server.listen(3000, () => console.log('Listening on port 3000'));

The server answered GET /hello with the path it received. Change the path in the request bar to /anything and send it: the reply follows, because this server handles every URL with one function.

When to choose Node.js

Choose Node.js when the work is mostly waiting on I/O (requests, databases, files) and you want one language on the frontend and backend. It is a weaker fit for long CPU-heavy calculations such as video encoding or large numeric work, because a long computation blocks the single JavaScript thread and every other request waits for it.