download_csv.js

$.post({
  url:      'makecsv.php',
  dataType: 'json',
  data:     {
    data: data
  }
}).done(function(response){
  if(response != ''){
    let uInt8List;
    let code = $('#result-output-encode').val();
    //エンコードしてBrobにセット
    if (code === 'sjis') {
      var codes = Encoding.stringToCode(response);
      var shiftJisCodeList = Encoding.convert(codes, 'sjis', 'Unicode');
      uInt8List = new Uint8Array(shiftJisCodeList);
    } else {
      uInt8List = response;
    }
    const blob = new Blob([uInt8List], { type: 'text/csv' });
    //リンクを作成してクリック
    const a = document.createElement('a');
    document.body.appendChild(a);
    a.href = window.URL.createObjectURL(blob);
    a.download = 'download.csv';
    a.click();
  }
}).fail(function( jqXHR, textStatus, errorThrown ){
  console.error( 'ERROR', jqXHR, textStatus, errorThrown );
}).always(function(response){
  //do nothing
});