Newtab Browser Flags

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JavaScript Content Script Example

Replaces your new tab page with a launcher that opens the extension's sidebar panel.

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.

Demonstrates browser-specific manifest keys (chromium:*, firefox:*) so a single manifest.json ships clean to multiple targets.

Try it locally

npx extension@latest create my-new-browser-flags --template new-browser-flags
cd my-new-browser-flags
npm install
npm run dev

A fresh browser window opens with the extension already loaded.

Project layout

src/
├── content/
│   └── content.js
├── images/
│   └── icon.png
├── newtab/
│   ├── index.html
│   ├── newtab.css
│   └── newtab.js
├── sidebar/
│   ├── index.html
│   └── sidebar.css
├── 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