Auto-Incrementing Columns on Google Spreadsheets

autoincrementing-col-appscript.js

function myFunction() {

  var AUTOINC_COLUMN = 0;
  var HEADER_ROW_COUNT = 1;
  
  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  var worksheet   = spreadsheet.getSheetByName("Sheet1");
  var rows        = worksheet.getDataRange().getNumRows();
  var vals        = worksheet.getSheetValues(1, 1, rows+1, 2);
  
  for (var row = HEADER_ROW_COUNT; row < vals.length; row++) {
    try {
      var id = vals[row][AUTOINC_COLUMN];
      Logger.log(id);Logger.log((""+id).length ===0);
      if ((""+id).length === 0) {
        // Here the columns & rows are 1-indexed
        worksheet.getRange(row+1, AUTOINC_COLUMN+1).setValue(row);
      }
    } catch(ex) {
      // Keep calm and carry on
    }
  }
}

I had to change line 8 to
var rows = worksheet.getDataRange().getNumRows() - 1;
in order to apply it properly to my spreadsheet. I had one header row, but the script was adding an ID to one row below my actual data.