What Is Node.js?

Introduction

JavaScript began as a language for web browsers. Node.js lets you run JavaScript outside the browser, on your own computer or on a server. That means you can use one language to build websites, command-line tools, APIs and much more. This course assumes you know basic JavaScript. The code is shown for reading, and the outputs come from real runs with Node 24.

The simple explanation

Node.js is a program that reads your JavaScript file and runs it, with extra tools for files, networks and the operating system.

Browsers give JavaScript things like document and window. Node does not have those. Instead it gives you:

• Files and folders (fs)
• Networking and web servers (http)
• Operating system information (os, process)
• Cryptography (crypto) and much more

It runs on the same V8 engine that powers Chrome, so the language itself is the same.

Install and check

Download the current LTS (long-term support) version from the official Node.js website, or use your system's package manager. Then in a terminal:

node --version
npm --version

You will see version numbers. LTS versions are the safest choice for real projects.

Your first script

Create a file named hello.js:

Run it:

node hello.js

Hello from Node!
5

There is no browser and no HTML. The output goes to your terminal.

Command-line arguments

process.argv is an array of the words you typed. The first two items are Node and the script, so your arguments start at index 2:

Running node greet.js Ada Lovelace prints:

You typed: [ 'Ada', 'Lovelace' ]

The REPL

Type just node and press Enter to open the REPL (read, evaluate, print loop), an interactive prompt:

> 2 + 3
5
> [1, 2, 3].map((n) => n * 2)
[ 2, 4, 6 ]
> .exit

It is perfect for trying small ideas. Type .exit or press Ctrl+D to leave.

What people build with Node

• Web servers and APIs, which you will build in this course
• Command-line tools, such as build scripts
• Real-time apps, such as chat
• Tools for front-end developers, such as bundlers and test runners

Node is good at work that waits for things, such as files and networks, because of how it handles asynchronous code. You will see that in the third lesson.

Try it yourself

Write a script that prints the arguments you give it in upper case, joined with commas. Run it with node shout.js one two three.

Summary

• Node.js runs JavaScript outside the browser, with built-in modules for files, networking and the system.
• Run a script with node file.js, read arguments from process.argv, and experiment in the REPL.
• Choose an LTS version for real projects.