Auto Increment (Sequence) in Google Sheets / Google Apps Script

gapp.js

function onEdit(e) {
  // Auto increment number column name
  var aiColumnName = 'A';
  // The column that is tested, seq numbers will be
  // given if this column is not empty. 
  var requieredColName = 'B'

  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  var worksheet = spreadsheet.getActiveSheet();
  var aiColRange = worksheet.getRange(aiColumnName + '1:' + aiColumnName + '1000');
  var aiCol = aiColRange.getValues();
  var aiColIndex = aiColRange.getColumn();
  var reqCol = worksheet.getRange(requieredColName + '1:' + requieredColName + '1000').getValues();

  var maxSeq = 0;
  for (var i = 0; i <= aiCol.length; i++) {
    if (parseInt(aiCol[i], 10) > maxSeq) { maxSeq = aiCol[i]; }
  }

  for (var i = 0; i <= aiCol.length; i++) {
    if (('' + reqCol[i]).length > 0 && ('' + aiCol[i]).length === 0) {
      maxSeq++;
      worksheet.getRange(i + 1, aiColIndex).setValue(maxSeq);
    }
  }
}