JSHint
A tool that helps to detect errors and in your JavaScript code
...The project aims to help JavaScript developers write complex programs without worrying about typos and language gotchas. Any code base eventually becomes huge at some point, so simple mistakes, that would not show themselves when written, can become show stoppers and add extra hours of debugging. So, static code analysis tools come into play and help developers spot such problems. JSHint scans a program written in JavaScript and reports about commonly made mistakes and potential bugs. The potential problem could be a syntax error, a bug due to an implicit type conversion, a leaking variable, or something else entirely.