whole_func_string: function handleOnFilter(file, filterName) {
      let usedFilters = file.filters;
      usedFilters || (usedFilters = file.filters = []);
      if (!usedFilters.includes(filterName)) {
          usedFilters.push(filterName);
      }
  }
func_code_string: function handleOnFilter(file, filterName) {
      let usedFilters = file.filters;
      usedFilters || (usedFilters = file.filters = []);
      if (!usedFilters.includes(filterName)) {
          usedFilters.push(filterName);
      }
  }
func_documentation_string: On filter handler
  
  @inner
  @param {Object} file the template file
  @param {string} filterName the filter name