Injects a small styled badge into every web page you visit, with a Sass modules options page that moves it to the left or right edge.

What you'll see: A small UI injected into any web page, isolated in a Shadow DOM so site styles don't bleed through.

How it works: A content script mounts a JavaScript UI inside a Shadow DOM and applies scoped styles so the host page can't bleed through. Styles flow through Sass + CSS Modules.

Sass-flavored CSS Modules. Combines .module.scss files with class-name hashing for fully isolated, nested styles.

npx extension@latest create my-content-sass-modules --template content-sass-modules
cd my-content-sass-modules
npm install
npm run dev

A fresh browser window opens with the extension already loaded.

src/
├── content/
│   ├── scripts.js
│   └── styles.module.scss
├── images/
│   ├── icon-128.png
│   ├── icon-16.png
│   ├── icon-32.png
│   ├── icon-48.png
│   ├── icon-64.png
│   └── icon.png
├── options/
│   ├── index.html
│   ├── scripts.js
│   └── styles.module.scss
├── background.js
└── manifest.json

Cloned this repo instead? The examples ship without npm scripts, so run Extension.js directly from the example directory. Run npm install first when the example declares dependencies.

Run the extension in development mode. Target a browser with --browser:

npx extension@latest dev .                  # Chromium (default)
npx extension@latest dev . --browser=chrome
npx extension@latest dev . --browser=edge
npx extension@latest dev . --browser=firefox

Build for production:

npx extension@latest build .                # Chromium (default)
npx extension@latest build . --browser=firefox
npx extension@latest build . --browser=edge

Preview the production build with the bundled browser:

npx extension@latest preview .

This template ships an end-to-end check (template.spec.ts) validated by the examples-repo CI on every commit.

• Extension.js docs
• Templates index
• GitHub: extension-js/extension.js