BabelExt is a cross browser boilerplate/library for extension development
JavaScript HTML Shell Batchfile


BabelExt is essentially obsolete at this point, as Firefox now supports WebExtensions, Opera is now Chromium-based, and even Microsoft Edge has adopted Chrome's format.

This was an incredibly fun/interesting exercise, but it will no longer be maintained at this point! Thanks to all of those who used and provided feedback over the years.


MIT (X11) license. See LICENSE.txt

What is BabelExt?

BabelExt is a library (or perhaps more of a boilerplate) meant to simplify the development of cross-browser "userscript" style extensions for the following browsers:

  • Chrome
  • Firefox
  • Opera
  • Safari

You can see a "kitchen sink" demonstration at

Who is BabelExt for?

It's likely that BabelExt will appeal most to either new extension developers, or to the existing pool of Greasemonkey script developers - which is how I got started with extension development. The transition from Greasemonkey development to browser extension development wasn't too difficult - but there are a few nuances in each browser that are a bit of a pain to circumvent if you're in a "Greasemonkey mindset"

BabelExt is definitely more suited for developers wanting to create "content enhancement" extensions that enhance websites. It's not made for creating addons such as AdBlock Plus, etc.

What does BabelExt do to help me?

BabelExt takes care of commonly used functionality that you might want to perform in content-script-like extension. Some of these things seem simple, but each browser has its own function calls and way of working, including, but not limited to:

  • Accessing and controlling tabs (i.e. opening a link in a new one and choosing if it's focused)
  • Cross domain http requests (extensions require)
  • Storing data (using HTML5 localStorage or similar/equivalent engines)
  • Triggering notifications (desktop or browser, depending on the browser's particular level of support)
  • Adding URLs to history (to mark links as visited)
    • Note: this is a bit of a hack in all non-Chrome browsers...
  • Adding CSS to the page

What does BabelExt NOT do?

Well, a lot! Most things, in fact! However, I have some clear goals, and some clear things I'm probably not interested in adding to BabelExt... Specifically, it's geared towards assisting in content script development - extensions that enhance specific websites or functionality on the web. For this reason, functionality that is not supported by one or more of the 4 BabelExt browsers (Chrome, Firefox, Opera, Safari) may not be added to BabelExt.

BabelExt also isn't meant to handle building each browser's native settings consoles/panels, etc. They're just too different from each other to try and abstract into a nice little package, and with the 4 supported browsers all handling modern HTML/CSS/Javascript so well - it makes sense (to me, anyhow) to build settings consoles and the like using those technologies.

That's what I did with Reddit Enhancement Suite, and it has worked rather well. I am considering adding the automatic form rendering code from RES into BabelExt, but I will need to devote some thought to how to make it more universally useful.

Great, now how do I get started?

First, download all of the source from Github and put it together within a folder.

In Windows, run makelinks.bat to create symlinks to extension.js - these links are not handled by github, which is why you unfortuntately have to make them yourself. NOTE: You may need to open a command prompt as Administrator for this batch file to work.

In UNIX-based OSes, you can run Note that this will make hardlinks.

IMPORTANT OPERA NOTE: Note that the Opera js file has .user.js in it - that's because without this, @include and @exclude directives will be ignored and your script will run on every page on the internet!

IMPORTANT SAFARI NOTE: Safari has a "security feature" that is not documented, gives no user feedback at all, and can be a HUGE time sink if you don't know about it! If you have any files in your extension folder that are symlinks, Safari will silently ignore them. With Safari, a hard link will work, but a symbolic link will not. If you made the links yourself instead of using the batch file, and your extension is doing nothing at all in Safari, double check that!

One last Safari quirk: if the directory does not end in ".safariextension", it will not be recognized by Safari. Don't remove that from the name!

Instructions for loading/testing an extension in each browser




  • Click Tools -> Extensions -> Manage Extensions

  • Find the config.xml file in the Opera directory of BabelExt, and drag it to the Extensions window

  • You're good to go! If you just want to try out the BabelExt kitchen sink demo, navigate to

  • Further Opera development information can be found at