code: function(target, attribute) {
              var self    = this,
                  value   = '';
  
              value = target.dom.el.getAttribute('data-' + attribute);
  
              if (value === null) {
                  if (self.config.debug.showWarnings) {
                      // Encourage users to assign values to all targets to avoid erroneous sorting
                      // when types are mixed
  
                      console.warn(mixitup.messages.warningInconsistentSortingAttributes({
                          attribute: 'data-' + attribute
                      }));
                  }
              }
  
              // If an attribute is not present, return 0 as a safety value
  
              return self.callFilters('valueGetAttributeValue', value || 0, arguments);
          }
docs: Reads the values of any data attributes present the provided target element
  which match the current sort command.
  
  @private
  @instance
  @since   3.0.0
  @param   {mixitup.Target}    target
  @param   {string}            [attribute]
  @return  {(String|Number)}