For example if you create an alert box with ExtJS you can always add a no-op classname to it. This way you can write your Selenium test to do class based look ups and then verify the contents of the child nodes within.
I agree. The Selenium project could really use some volunteers to look at the major JS frameworks out there and help make them more testable with Selenium (or WebDriver, which Selenium is merging with in Se 2.0).
Might be a good start.
I am developing a site
Everything that followed that utterance was a bad idea. At that point, you're not using javascript for the portability, light weight OR "scripting". You should have used any "applet" RIA technology out there and you would have been fine; java, flash, silverlight.
As regards the original question, I'm sorry I can't help. You should try the ExtJS website, they have a vibrant community there.
It is a wrong perception that JS needs to be used only for portability, light weight OR "scripting". AJAX frameworks exist for a reason.
The reason being they are far more scalable, modifiable, flexible, compatible and most importantly they have seamless integration with any server side technology. Also the development is possible to break into distinct task and get them outsourced/shared as required.
I am not claiming that other technologies cant do all this but, JS based AJAX frameworks make your life easy.