01 · Setup & First Program¶

Install Node.js¶

Download the LTS release from nodejs.org or use your OS package manager:

# macOS (Homebrew)
brew install node

# Ubuntu/Debian
sudo apt install nodejs npm

# Windows: use the installer from nodejs.org and check "Add to PATH"

Verify the install:

node --version
# v20.x.x
npm --version
# 10.x.x

The Node REPL¶

The REPL (Read-Eval-Print Loop) is an interactive JavaScript shell — great for quick experiments:

node
> 2 + 2
4
> console.log("hello")
hello
> .exit

The browser console¶

Every browser ships a JavaScript console too. Open your browser's DevTools (usually F12 or Cmd+Option+I) and switch to the "Console" tab — it's the same JavaScript engine that runs the pages you visit, and a quick way to test DOM code (covered in Module 7).

// Paste directly into the browser console:
console.log("hello from the browser");
document.title; // reads the current tab's title

Your first script¶

Create hello.js:

// hello.js
function greet(name) {
  return `Hello, ${name}!`;
}

console.log(greet("world"));

Run it with Node:

node hello.js
# Hello, world!

Running JavaScript in the browser¶

Create index.html alongside hello.js:

<!-- index.html -->
<!DOCTYPE html>
<html>
  <head>
    <title>First Program</title>
  </head>
  <body>
    <script src="hello.js"></script>
  </body>
</html>

Open index.html in a browser and check the console — greet runs there too, because the same JavaScript works in both environments (as long as it doesn't use Node-only or browser-only APIs).

Choosing an editor¶

Any of these work well for this program: VS Code (free, huge JavaScript/Node extension ecosystem), WebStorm (free for non-commercial use), or even a plain text editor plus the terminal. Pick one and move on — the editor matters far less than practice.

How It Actually Works¶

When you run node app.js, Node doesn't just "read your code and do it." It boots the V8 engine (the same JS engine inside Chrome), which first parses your source into an Abstract Syntax Tree (AST), then compiles it — not to machine code immediately, but through Ignition, V8's bytecode interpreter. Only functions that run often enough ("hot" functions) get promoted to TurboFan, V8's optimizing JIT compiler, which emits real machine code specialized for the shapes of data you've actually passed in. This is why the first run of a script is always interpreted bytecode, and why a function can get faster the more it's called — V8 is learning about it as it goes.

The node binary itself is a thin C++ shell around V8 plus libuv, the library that gives Node its event loop, thread pool, and non-blocking file/network I/O. When you later write console.log, that's not part of the JS language at all — it's a host API that Node injects into the global object, backed by libuv writing to a file descriptor. Understanding this split — V8 runs pure JavaScript, Node/the browser bolt on I/O and timers — explains almost every "why does this work differently here" question you'll hit later in the course.

🔀 See this in another language¶

Exercise¶

Write a script greet-many.js that defines an array of three names and prints a greeting for each one using the greet function above.