testing question

gistfile1.js

<snipped>   
  var watchFolderTree = function (fPath, fn) {
    if (typeof fn !== 'function') {
      throw {
        name: 'TypeError', 
        message: 'fn must be a function'
      };
    }

   //Do lots of things to watch the folder tree
   //How to test all this stuff?

   return fn && fn();
  };


  exports.watch =  function(reqPath, fn) {
    var nPath = path.resolve(reqPath);

    path.exists(nPath, function onExistsReturn(exists) {
      if (!exists) {
        nPath = nPath || '';
        return fn && fn('Path does not exist: ' + nPath);
      }

      watchFolderTree(nPath, fn);
    });
  };