code: function onSortableClick( event ) {
    if ( this.ui.sortButton ) {
      this.ui.sortButton.classList.remove( this.sortClass );
    }
    if ( this.ui.sortButton === event.target ) {
      this.sortDirection = ~this.sortDirection;
    } else {
      this.ui.sortButton = event.target;
      this.sortColumnIndex = this.getColumnIndex();
      this.sortDirection = DIRECTIONS.UP;
    }
    // The active sort class is changing when the sort direction changes.
    this.ui.sortButton.classList.add( this.sortClass );
    this.updateTable();
  
    return this;
  }
docs: /* eslint-enable complexity 
  Function used as callback for the sortable click event.
  
  @param {Event} event - DOM event.
  @returns {Object} - TOOD: Add description.