Is there something like Compass but for javascript? I'm interested in a tool where it will take the main js files and all the other files that are included and compress it to a new js file with all the code on 1 line as soon as you save your changes in whatever file you are working.
You're looking for a Javascript minifier. There are a whole bunch of them to choose from. Try this one or this one, or google for "javascript minifier" for more.
Note that in addition to simply putting all the code on one line, these tools also rename your variables and functions to shorter names, and do a few other tricks to make the code as tiny as possible. The end result should be functionally identical to your original code, but it won't be exactly the same as the code you wrote.