A JavaScript runtime (built on Chrome's V8 engine) for running JS outside the browser, servers, CLIs, build tools.

Setup

Install via nvm (Node Version Manager) rather than a system package, so you can switch versions per project:

nvm install --lts
nvm use --lts
node -v
npm -v

Running a script

node index.js

package.json & npm basics

npm init -y
npm install express
npm install -D nodemon   # dev-only dependency

{
  "name": "my-app",
  "type": "module",
  "scripts": {
    "start": "node index.js",
    "dev": "nodemon index.js"
  },
  "dependencies": { "express": "^4.19.0" }
}

"type": "module" switches the project to ES module syntax (import/export); omit it to stay on CommonJS (require/module.exports), don't mix the two styles in one file.

A minimal HTTP server (no framework)

import { createServer } from "node:http";

const server = createServer((req, res) => {
  res.writeHead(200, { "Content-Type": "application/json" });
  res.end(JSON.stringify({ message: "hello" }));
});

server.listen(3000, () => console.log("Listening on :3000"));

With Express (the common default)

import express from "express";
const app = express();
app.use(express.json());

app.get("/users/:id", (req, res) => {
  res.json({ id: req.params.id, name: "Ada" });
});

app.post("/users", (req, res) => {
  const { name } = req.body;
  res.status(201).json({ id: Date.now(), name });
});

app.listen(3000, () => console.log("Listening on :3000"));

File system & environment

import { readFile } from "node:fs/promises";
const data = JSON.parse(await readFile("./config.json", "utf-8"));

const port = process.env.PORT ?? 3000;

Load .env files with the dotenv package (npm i dotenv, then import "dotenv/config" at the top of your entry file) rather than hardcoding secrets.

Popular libraries

Common gotchas

• Uncaught errors in async callbacks (not awaited, no .catch()) crash the process silently in some setups, always handle rejected promises.
• CPU-heavy synchronous work blocks the entire event loop (Node is single-threaded for JS execution), offload it to worker_threads or a separate service.
• npm install without a lockfile committed leads to different dependency versions across machines, always commit package-lock.json.