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Running JavaScript

How to run JavaScript code​

Unlike Python, which requires installation, JavaScript comes built into every web browser. However, to run JavaScript outside the browser, you'll need Node.js installed.

note

There are other JavaScript runtimes besides Node.js, like Bun and Deno. For this guide, though, we'll stick with Node.js because it’s the most widely used and best supported place to start.

This guide shows you three main ways to run JavaScript:

  1. Browser console — quick experimentation
  2. JavaScript files — writing and saving programs
  3. Node.js — running JavaScript from the terminal

JavaScript in the browser console​

Every modern web browser has a built-in JavaScript console where you can type and run JavaScript code immediately.

Opening the console​

  • Chrome/Edge: Press F12 or Ctrl+Shift+I (Windows/Linux) or Cmd+Option+I (Mac), then click the "Console" tab
  • Firefox: Press F12 or Ctrl+Shift+K (Windows/Linux) or Cmd+Option+K (Mac)
  • Safari: Enable Developer menu first (Preferences → Advanced → "Show Develop menu"), then Cmd+Option+C

Running code in the console​

Once the console is open, you can type JavaScript code and press Enter to run it:

console.log("Hello, World!");
// Output: Hello, World!

const name = "Alice";
console.log(name);
// Output: Alice

The console is perfect for:

  • Quick experiments
  • Testing small snippets of code
  • Debugging (seeing what your code actually does)

Note: This guide focuses on core JavaScript, not browser-specific features. The console is useful for learning, but we won't cover DOM manipulation or other browser APIs here.

JavaScript in a .js file​

For real programs, you'll write JavaScript code in files with the .js extension.

Creating your first JavaScript file​

  1. Create a new file called hello.js
  2. Add this code:
console.log("Hello, World!");
console.log("This is my first JavaScript program!");
  1. Save the file

Running the file in Node.js​

Open your terminal and navigate to the directory containing hello.js, then run:

node hello.js

You should see:

Hello, World!
This is my first JavaScript program!

Running in a browser (HTML file)​

You can also run JavaScript files in a browser by embedding them in an HTML file:

<!DOCTYPE html>
<html>
<head>
  <title>My First JavaScript</title>
</head>
<body>
  <script src="hello.js"></script>
</body>
</html>

Open the HTML file in a browser, and the JavaScript will run. However, for learning core JavaScript (without browser features), Node.js is recommended.

Using node to run JavaScript​

Node.js is a JavaScript runtime that lets you run JavaScript outside of a browser. It's essential for learning JavaScript as a programming language.

Installing Node.js​

  1. Visit nodejs.org
  2. Download the LTS (Long Term Support) version
  3. Follow the installation instructions for your operating system

Verifying installation​

After installing, open your terminal and type:

node --version

You should see something like v20.10.0 (the exact version doesn't matter, as long as it's recent).

Running JavaScript files with Node​

Once Node.js is installed, you can run any .js file:

node filename.js

Example:

// greet.js
const name = "Alice";
console.log(`Hello, ${name}!`);
node greet.js
# Output: Hello, Alice!

Running JavaScript interactively​

Node.js also provides an interactive REPL (Read-Eval-Print Loop) similar to Python's interactive mode:

node

This opens an interactive session where you can type JavaScript code:

> const x = 5;
undefined
> x * 2
10
> console.log("Hello!")
Hello!
undefined
> .exit

Press Ctrl+C twice or type .exit to quit.

console.log() As your primary debugging tool​

The console.log() function is your most important tool for seeing what your code is doing. It prints values to the console (browser console or terminal).

Basic usage​

console.log("Hello, World!");
console.log(42);
console.log(true);

Output:

Hello, World!
42
true

Logging variables​

const name = "Alice";
const age = 30;
console.log(name);      // Alice
console.log(age);       // 30
console.log(name, age); // Alice 30

Debugging with console.log()​

When learning or debugging, console.log() helps you see what's happening:

const x = 5;
const y = 10;
console.log("x is:", x);  // x is: 5
console.log("y is:", y);  // y is: 10

const sum = x + y;
console.log("sum is:", sum); // sum is: 15

You'll use console.log() constantly as you learn JavaScript. It's the JavaScript equivalent of Python's print().

Running code examples in this guide​

Throughout this guide, code examples can be run in two ways:

  1. Copy into a .js file and run with node filename.js
  2. Paste into your browser's console (though some examples won't work without Node.js features)

For the best learning experience, create .js files and run them with Node.js. This matches how you'll write JavaScript in real projects.