document.getElementById("changeCol").addEventListener('click', function (e){
    theGrid.columns.clear(); //theGrid는 FlexGrid의 인스턴스
    theGrid.allowAddNew = true;

    ['country','active','downloads'].forEach(column=>{
      var col = new wijmo.grid.Column({
        binding: column,
        header: column,
      });
      theGrid.columns.push(col)
    })
  })