code: function getIdMapping(idColumnNames, columns) {
    var idColumns = getIdColumns(idColumnNames, columns);
    if (idColumns.length === 0) {
      return {};
    }
    return idColumns[0].values.reduce(function(result, value, rowNumber) {
      var idString = getIdStringForRowNumber(idColumns, rowNumber);
      if (!defined(result[idString])) {
        result[idString] = [];
      }
      result[idString].push(rowNumber);
      return result;
    }, {});
  }
docs: Both arguments are required.