Install & Your First Program

One idea saves a lot of confusion before you write any JavaScript: the same language runs in two completely different places, and they don't have the same tools. Get this straight now and a whole category of "why doesn't this work?" disappears.

The mental model: a language vs. a place to run it

JavaScript is just a language - grammar and rules for writing instructions. By itself it doesn't do anything; it needs a program to read and carry out those instructions. That program is called a runtime (or engine).

There are two runtimes you'll meet first:

• The browser (Chrome, Firefox, Safari, Edge). Every browser has a JavaScript engine built in - this is JavaScript's original home, and it exists to make web pages interactive. Your code can touch the page: change text, respond to clicks, draw things.
• Node.js - a runtime that takes the browser's JavaScript engine and runs it on your computer outside any web page, so JavaScript can do what a normal program does: read files, run a web server, work with your file system.

📝 Terminology. A runtime is the program that executes your JavaScript. Same language, different runtime, different powers.

⚠️ The two are not interchangeable. The browser has objects like window and document (the web page) that do not exist in Node. Node has tools like fs (the file system) that do not exist in the browser - a web page on a stranger's site has no business reading your hard drive. Copy browser code into Node and see document is not defined? Not a bug - that tool doesn't exist there.

Install Node.js

You'll do most of your early learning in Node, since running a file from the terminal is the simplest loop there is.

The simple way: go to nodejs.org and download the LTS version ("Long Term Support" - the stable one most projects use). Run the installer and accept the defaults.

The flexible way (recommended once you're comfortable): install nvm ("Node Version Manager") to install and switch between multiple Node versions - real projects often pin a specific version, and nvm makes that painless. On Windows, use nvm-windows.

Either way, confirm it worked by asking Node its version:

$ node --version
v24.11.0

What just happened: node --version printed the installed version. The exact number will differ - what matters is a v and some numbers, not command not found. Getting that error means Node isn't on your PATH yet; reopening your terminal (or rebooting) usually fixes a fresh install.

📝 Terminology. A flag is an extra option passed to a command, usually starting with -- (or a single -). --version means "tell me your version and exit."

Your first program: a file Node runs

Create a file called hello.js (.js is the convention for JavaScript files) with one line in it:

console.log("Hello, JavaScript!");

What just happened: console.log prints its argument to the output. (It works in both runtimes - in Node it prints to your terminal, in the browser to the browser's console. One of the few tools both share.)

Now run the file with Node:

$ node hello.js
Hello, JavaScript!

What just happened: Node read the file top to bottom, ran the one instruction, and printed the result. That's the loop you'll repeat thousands of times: edit a .js file, run it with node, read the output.

💡 Key point. node hello.js means "Node, run this file." Nothing to compile, no build button - Node reads your source directly and runs it.

The other runtime: the browser console

Running JavaScript in a browser needs no install - it's already there. Open any web page, then open the developer console:

• Chrome / Edge: press F12, or Ctrl+Shift+J (Windows/Linux) / Cmd+Option+J (Mac).
• Firefox: press F12, or Ctrl+Shift+K (Windows/Linux) / Cmd+Option+K (Mac).
• Click the Console tab.

Type JavaScript at the prompt and press Enter to run it immediately:

console.log("Hello from the browser!");
2 + 2

What just happened: The console is a live, one-line-at-a-time runtime. console.log printed your text; the bare line 2 + 2 got evaluated and the console echoed its result, 4, automatically - a great scratchpad. (A .js file run by Node shows no output for a bare 2 + 2; that's a console convenience, not a language feature.)

🪖 War story. Nearly every JavaScript developer has opened the browser console expecting Node behavior and gotten a confusing result - or pasted Node code into the console and watched it complain that require or fs doesn't exist. It's the two-runtimes thing again. When code surprises you, ask: which runtime am I in, and does it have the tool I just used?

Recap

1. JavaScript is a language; a runtime is what runs it. The two you start with are the browser and Node.js.
2. The runtimes have different tools. The browser has window/document; Node has fs. Code written for one can fail in the other - that's expected, not broken.
3. Install Node from nodejs.org (LTS) or via nvm, and confirm with node --version.
4. Run a file with node hello.js; console.log(...) prints output in both runtimes.
5. The browser console is a live JavaScript scratchpad - open it with F12 and the Console tab.

Next: values, the types they come in, and the ways to name and combine them.

← Guide overview · Phase 2: Syntax, Values & Types →