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Browser Extensions Development (Chrome & Edge) · Lesson

Your First 'Hello World' Extension

Create a minimal 'Hello World' extension, covering the manifest file, basic structure, and loading into the browser.

Your First 'Hello World' Extension is a free Browser Extensions Development (Chrome & Edge) lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Browser Extensions Development (Chrome & Edge) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Your First Extension: Hello World

Time to build your first extension — a classic Hello World popup that greets you when you click its toolbar icon. The core Manifest V3 shape.

Core Extension Components

Every extension needs a few key files: manifest.json (the blueprint), popup.html (the popup content), and an icon. We'll build them step by step.

Manifest.json: The Blueprint

The manifest.json is the most important file — JSON metadata like name, version, and permissions, in the structure Manifest V3 expects.

Essential Manifest Fields

Start your manifest with three required fields: manifest_version 3, a name, and a version. The snippet below shows the minimal shape.

{ "manifest_version": 3,
  "name": "Hello World Extension",
  "version": "1.0"
}

Adding Description & Icons

Add a description and an icons map so users understand and recognize your extension. Provide several sizes like 16, 48, and 128 pixels.

{ "manifest_version": 3,
  "name": "Hello World Extension",
  "version": "1.0",
  "description": "A simple 'Hello World' extension for CoddyKit.",
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon128.png"
  }
}

Defining the Popup with 'action'

The action field defines the toolbar button: default_popup points to your popup HTML, and default_icon sets the toolbar image.

{ "manifest_version": 3,
  "name": "Hello World Extension",
  "version": "1.0",
  "description": "A simple 'Hello World' extension for CoddyKit.",
  "icons": { "16": "icon16.png" },
  "action": {
    "default_popup": "popup.html",
    "default_icon": { "16": "icon16.png" }
  }
}

Creating popup.html

Create popup.html — a plain HTML file holding your Hello World message. Keep it small and light, since it shows in a tiny popup window.

<!DOCTYPE html>
<html>
<head>
  <title>Hello World Popup</title>
  <style>
    body { font-family: sans-serif; padding: 15px; width: 150px; }
    h1 { font-size: 1.2em; color: #333; margin: 0; }
  </style>
</head>
<body>
  <h1>Hello, CoddyKit!</h1>
</body>
</html>

Your Extension's Folder Structure

Your folder should now hold manifest.json, popup.html, and the icons, all directly inside — a clean layout the browser can load correctly.

Loading Your Extension (Chrome)

Load it in Chrome or Edge: open chrome://extensions, flip on Developer mode, click Load unpacked, and pick your folder. It joins the list!

Test Your New Extension!

Spot your extension's icon in the toolbar and click it — a popup appears reading "Hello, CoddyKit!" from your popup.html. First extension done!

Quick Check: Manifest V3

Which field is absolutely essential to declare the manifest format version in manifest.json for a Manifest V3 extension?

Recap: Your First Extension

You built and loaded a real extension: a manifest with key fields, an action popup, a popup.html, all run via Developer mode. Keep tinkering!

Frequently asked questions

Is the “Your First 'Hello World' Extension” lesson free?

Yes — the full text of “Your First 'Hello World' Extension” is free to read here on the web, and the Browser Extensions Development (Chrome & Edge) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Browser Extensions Development (Chrome & Edge) course, upgrade to CoddyKit PRO.

What will I learn in “Your First 'Hello World' Extension”?

Create a minimal 'Hello World' extension, covering the manifest file, basic structure, and loading into the browser. You practise Browser Extensions Development (Chrome & Edge) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Browser Extensions Development (Chrome & Edge)?

No prior experience is required. Browser Extensions Development (Chrome & Edge) on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Your First 'Hello World' Extension” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Browser Extensions Development (Chrome & Edge) lesson?

Yes. Every Browser Extensions Development (Chrome & Edge) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. What are Browser Extensions?
  2. Setting Up Development Environment
  3. Your First 'Hello World' Extension
  4. Anatomy of an Extension Project
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