I ran into an annoying problem: the following code gives a warning in Visual Studio.
This is not the standard behavior, and it is just happening randomly, and in such a situation just renaming the file and then building will help, (and it is possible that even without building it will be solved), and you can then revert back.
You can also completely disable warnings on Java Script under Tools -> Options -> Text Editor -> JScript -> Miscellaneous.
Of course in this case you will miss the Java Script warnings, but at least you will be able to concentrate on the server side warnings.