jsc is a decompiler or a cross compiler if you will. It is not a source-code parser nor an IDE. It is a command line utility like any other compiler. jsc was originally an anagram for csharp to javascript.
No need to install anything on your computer. Use the easy upload interface to convert your file. Your original file will not be affected, and you can choose your own file name for the converted file which will instantly download to your computer.