Main-world Content Script

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Content Script in MAIN World Example

Injects a small UI into every web page from both the extension and the page's own JavaScript world.

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.

Loads the content script in the page's MAIN world (Chromium-only). Useful when your script needs direct access to page-side globals or classes that the isolated world cannot reach. Firefox does not support MAIN-world content scripts, so this template is gated to Chromium targets.

Try it locally

npx extension@latest create my-content-main-world --template content-main-world
cd my-content-main-world
npm install
npm run dev

A fresh browser window opens with the extension already loaded.

Project layout

src/
├── content/
│   ├── utils/
│   │   ├── constants.js
│   │   └── create-badge.js
│   ├── isolated-one.js
│   ├── isolated-two.js
│   ├── scripts.js
│   └── styles.css
├── images/
│   └── icon.png
├── background.js
└── manifest.json

Commands

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.

dev

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

Build for production:

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

preview

Preview the production build with the bundled browser:

npx extension@latest preview .

Tests

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

Learn more

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