Introduction to JavaScript

Last reviewed May 28, 2026 Content v20260528

Track mode

client_javascript

Means

In-browser JS

Reading

~2 min

Level

beginner

This lesson

An orientation to this JavaScript lesson—scope, vocabulary, and what you will practice next.

You need a clear map of the JavaScript track so later lessons do not feel like isolated tricks.

You will apply Introduction to JavaScript in contexts like: Browsers, Node.js services, edge workers, and tooling ecosystems (bundlers, test runners).

Run JavaScript in the in-browser sandbox, use the terminal output panel, and verify with MCQs. Also open the interview prep blocks.

Start here at the beginning of the javascript track before skipping ahead.

How this JavaScript track works

36 lessons · 108 MCQs — fundamentals → DOM → async → patterns → interview wrap-up.
Playground — client_javascript: edit code, click Run in browser, use console.log and printOutput(...).
Prerequisites — HTML and CSS (or HTML & CSS) before DOM lessons.
Next tracks — TypeScript, React, Node.js, Cybersecurity for secure shipping.

Install on your device (macOS, Linux, Windows)

Lessons run in your browser on this site—install a modern browser and optional editor for local projects.

macOS

Use Safari (preinstalled) or install Google Chrome / Firefox.
Optional editor: VS Code (brew install --cask visual-studio-code).
Open DevTools with ⌥⌘I (Chrome/Edge) or ⌥⌘C (Safari Web Inspector).

Linux

Install Chromium or Firefox: sudo apt update && sudo apt install -y chromium-browser firefox (Debian/Ubuntu; package names vary by distro).
Fedora: sudo dnf install -y chromium firefox.
Optional editor: VS Code from code.visualstudio.com or sudo snap install code --classic.

Windows

Install Microsoft Edge or Chrome.
Optional editor: VS Code (winget install Microsoft.VisualStudioCode).
Open DevTools with F12 or Ctrl+Shift+I.

Verify: Open any lesson playground and click Run—output appears without installing a compiler.

JavaScript is the programming language of the web: interactivity, validation, APIs, and dynamic UI without full page reloads.

Prerequisites and track map

Complete HTML and CSS (or HTML & CSS) first—you will manipulate the DOM those tracks teach.

Fundamentals — values, control flow, functions, scope
Browser APIs — DOM, events, forms, fetch preview
Modern JS — promises, async/await, modules, array methods
Next steps — TypeScript, React, Node.js

Click Run in browser. Code executes in a sandboxed iframe. Use console.log and printOutput(...) for the output panel.

First script

const msg = 'Hello';
console.log(msg);
printOutput(msg);

Important interview questions and answers

Q: Why learn JavaScript?
A: Every interactive browser UI and most full-stack apps need it.
Q: Where does lesson code run here?
A: In a sandboxed browser iframe via client_javascript profile.

Self-check

Name two prerequisite tracks.
What does printOutput do in the playground?

Challenge

Hello plus one log

Run the default script.
Change the greeting string.
Add a second console.log line.

Done when: printOutput shows your updated values.

Challenge

Change one line and re-run

Run the default intro script.
Change the greeting string.
Log an extra value with console.log.

Done when: printOutput shows your updated object.

Interview prep

Where does code run?: In a sandboxed browser iframe (client_javascript).
Prerequisites?: HTML and CSS fundamentals.

Playground

Runs in your browser in a sandboxed frame. Backend runners appear when this track’s profile allows them.

Discussion

Past discussion is visible to everyone. Only logged-in users can post comments and replies.

Starter discussion topics

What would you log to verify this behavior?
What breaks if you run this before the DOM is ready?

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