I’m no Javascript expert and I couldn’t work out why it seemed to be so difficult to test to see if a function existed before I called it. In the end Google provided a better way though it wasn’t the most popular suggestion out there, to me it seems the most sensible way:

    if (window.myFunctionName) {
        myFunctionName();
    }

Where myFunctionName is the function you’re testing for, of course.