Google Apps Script for Spreadsheets - Gets the column number from a given sheet name and column name. The function assumes the column names are on the first row. The resulting column number is zero-based (Column A -> 0)

column_number_by_name.gs

function columnNumberByName(sheetName, ColumnName) {
  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = spreadsheet.getSheetByName(sheetName)
  
  var firstRowRange = sheet.getRange(1, 1, 1, sheet.getMaxColumns());
  var columnNames = firstRowRange.getValues()[0];
  
    for (var columnNumber in columnNames) {
      if (columnNames[columnNumber] == ColumnName) {
        return parseInt(columnNumber);
      }
    }
  
  throw 'No column named: ' + ColumnName + ' was found.';
}