I know there are lots of libraries that read byte codes that are written in Java. Does someone know of a byte code library that is implemented in Javascript?
Since javascript is typically run inside a browser, it generally cannot read the actual bytes out of files, which makes it less-than-ideal for reading java bytes. If you somehow got the byte codes encoded in a form that the javascript could read, what would you expect the library to do with it? Can you provide more details about what you're trying to do?
If you're looking to be able to write code in Java, and have it run inside a browser, take a look at GWT. It uses Java to recompile your byte-code into optimized javascript.
Edit
Based on your added comment, that you are hoping to "find out the classes and methods used in a jar file on my local disk":
Since javascript is unable to access files on a local disk (at least, without using ActiveX), the technology simply won't allow for this sort of thing. Is there a reason you wanted to use javascript for this, rather than java?
And please accept my apologies if it sounded like I was questioning your motives. I really just wanted to get enough information to be able to adequately answer your question.