Имеется онлайн-сервис по проверке JavaScript кода.

http://jshint.com

Сервис бесплатный, OpenSource.

Имеются так же плагины для текстовых редакторов и IDE.

Описание на английском:

JSHint is a community-driven tool to detect errors and potential problems in JavaScript code and to enforce your team's coding conventions. It is very flexible so you can easily adjust it to your particular coding guidelines and the environment you expect your code to execute in.

JSHint is open source and will always stay this way. If you really like JSHint and would like to donate to the project, you can do that via PayPal or GitTip.