code: function ( e ) {
              // Default context is `this` element
              var context = (selector ? matchElement( e.target, selector, true ) : this);
  
              // Handle `mouseenter` and `mouseleave`
              if ( event === "mouseenter" || event === "mouseleave" ) {
                  var relatedElement = (event === "mouseenter" ? e.fromElement : e.toElement);
  
                  if ( context && ( relatedElement !== context && !context.contains( relatedElement ) ) ) {
                      callback.call( context, e );
                  }
  
              // Fire callback if context element
              } else if ( context ) {
                  callback.call( context, e );
              }
          }
docs: Unique ID for each node event