OK, I realize you registered the event handlers inside the FMRadioTest function, so we could use module pattern here like this:

    (function() {
      ...
      window.addEventListener('load', init);
      window.addEventListener('unload', uninit);
    })();

but since the anonymous function is evaluated immediately when the js is loaded, so it's a good practice to initial the DOM node value, i.e. messageText, frequencyMin etc., inside the load event handler.