code: function computeFilterFn(filter) {
       if (filter) {
          if (typeof filter === 'string') {
             var regex = new RegExp(filter);
             return function(filePath) { return regex.test(filePath); }
          }
          else if (filter instanceof RegExp)
             return function(filePath) { return filter.test(filePath); }
          else   // must be a function itself
             return filter;
        }
        else  // none, return "pass all"
           return function() { return true; };
     }
docs: /* calculate file filter function