I was originally thinking about building two separate applications, but maintaining them would be time consuming. Instead I'm considering building a library abstraction layer that detects if the page is running jQuery or Prototype and then calls the appropriate methods. I'm not going to abstract the whole libraries, just the functionality applicable to my application — namely selectors, events & effect. The core of my app is under 500 lines of code, so there isn't too much I need to worry about.
So instead of calling
$('id') I would call
LA.$('id') (LA for Library Abstraction) which would call
$('id') in prototype and
$('#id') in query etc…
Does this sound sane? I can't think of any technical hurdles, although I would have expected someone to have attempted this before. I couldn't find anything similar in my searches.
I expect that if you support the libraries only partly then no one will choose to use it, as they would have to finish the support, and you may find that maintaining it will be a headache, as there will be requests to add more functionality.
If your application is so small, why not just switch to jQuery for it, and standardize on that, as MS has done.
You may run into problems with versions, as, if someone uses it, and they are using an older version of a library, and there was some API change, then they will be wanting you to add support for that library.
I believe Ext.Js does something similar. They have a concept of "adapters" which allow you to sit Ext.JS on top of any of the underlying libraries and it will just work. The key difference is that they are using a 1-to-1 model where you say which library you want to use and it joins the dots, whereas I believe you're trying to say "I don't know which one will be available but whatever one you find, go use it".
I don't think it's insane, but you might have some fun trying to work out which library to use, particularly if both are available.
Web development frameworks (Prototype, jQuery, etc.) themselves are designed to be abstractions on top of the various browsers that exist. You ask the framework to do one thing and it has the same result (ideally) regardless of the browser. So, here, you are proposing an abstraction on top of abstractions. Presumably because you want people to be able to use your tool regardless of what framework they have chosen for their site. While it sounds like an interesting idea, I would personally have to guess that it would not work out in the long run. Largely because you do not know what the future holds. Prototype used to be the number one use framework, now jQuery has surpassed it. Perhaps another one will be very popular in a year, what if you then want to support that framework? That can be a lot of conditional code that you have to add, and in addition, force being loaded into the browsers of those using your tool.
I say either pick a single framework to support and stick with it, or maintain separate libraries. Ideally, it would be really cool if you could write some sort of builder script. This would allow you to set framework rules in some list and the script would build separate scripts for each framework based on some core script and the rules list. I honestly am not sure how to best accomplish something like this but it would effectively give you that abstraction on abstraction power that you are looking for without it being visible to the end user.