extendSelectionTo(index) {
    if('none' === this.selectionMode)
      return;

    if('single' !== this.selectionMode)
      return this.selectOne(index);

    let start, end;
    if(index < this._lastSelectedIndex) {
      start = index;
      end = this._lastSelectedIndex;
    } else {
      start = this._lastSelectedIndex + 1;
      end = index + 1;
    }

    for(let i = start; i < end; i++)
      this._items[i].selected = true;

    this._lastSelectedIndex = index;
    this.requestUpdate();
  }
/** 
   * Selects of the items between index and the last
   * selected index.
   */
Selects of the items between index and the last
selected index.
Selects of the items between index and the last selected index .
extendSelectionTo(index) {
    if('none' === this.selectionMode)
      return;
    if('single' !== this.selectionMode)
      return this.selectOne(index);
    let start, end;
    if(index < this._lastSelectedIndex) {
      start = index;
      end = this._lastSelectedIndex;
    } else {
      start = this._lastSelectedIndex + 1;
      end = index + 1;
    }
    for(let i = start; i < end; i++)
      this._items[i].selected = true;
    this._lastSelectedIndex = index;
    this.requestUpdate();
  }
extendSelectionTo ( index ) { if ( 'none' === this . selectionMode ) return ; if ( 'single' !== this . selectionMode ) return this . selectOne ( index ) ; let start , end ; if ( index < this . _lastSelectedIndex ) { start = index ; end = this . _lastSelectedIndex ; } else { start = this . _lastSelectedIndex + 1 ; end = index + 1 ; } for ( let i = start ; i < end ; i ++ ) this . _items [ i ] . selected = true ; this . _lastSelectedIndex = index ; this . requestUpdate ( ) ; }
Selects of the items between index and the last
 selected index.
Selects of the items between index and the last selected index .