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Local by design

Extension.js runs on your machine: develop, run, and build extensions with the CLI. It ends at packaging on purpose. When a packaged build is ready for the stores, extension.dev, the platform that sponsors Extension.js, is the next step.

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Who this path is for

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Prerequisites

Extension.js requires Node.js 22.12 or newer. The CLI checks the runtime before any other code runs, and an older Node.js exits with code 1 and an upgrade message.
Check your version with node --version before you start.

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First successful outcome

1

Scaffold or run an extension project

Use extension create or choose a template to start from.
2

Launch dev and load the extension

Run extension dev and load the build output in a browser target.
3

Verify update behavior

Make one change and confirm the extension reloads.
4

Build production output

Run extension build for at least one browser.
First run. extension dev targets chromium by default and runs your extension in a version-pinned Chrome for Testing with an isolated profile, so it never touches the browser you use every day. When that browser is not on the machine yet, the first run asks to download it once and continues into the dev session as soon as it lands.Answer n and nothing is downloaded. Run npx extension install chromium whenever you want it, reach a browser you already have with --browser=edge or --browser=brave, pin any binary with --chromium-binary <path>, or start the dev server alone with --no-browser. A non-interactive shell, CI included, is never asked: it prints the install command and stops. See why Extension.js downloads a browser and skip the managed download.
  1. Get started immediately
  2. Create your first extension
  3. Templates
  4. Commands overview

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Next steps