Constructor inside if-statement returns Uncaught Type error

If I define the following constructor and method, I can call deck.tracker = new ActivityTracker(); without error:

function ActivityTracker() {}

    ActivityTracker.prototype = {
        trackEvent: function () {}
    }

If I add the code inside an if-statement (to limit the code to a local server):

  if (window.location.href.indexOf("http://0.0.0.0") == 0 || 
   window.location.href.indexOf('http://local') == 0) {

    function ActivityTracker() {}

    ActivityTracker.prototype = {
        trackEvent: function () {}
    }
};

the console returns: Uncaught TypeError: ActivityTracker is not a constructor. I have made sure, that the if-condition returns true.

Any idea what could be the cause?