docs: private static String parseFormat(List<? extends TargetExpr> args) {
      String numberFormatType = !args.isEmpty() ? args.get(0).getText() : "'" + DEFAULT_FORMAT + "'";
  
      if (!JS_ARGS_TO_ENUM.containsKey(numberFormatType)) {
        String validKeys = Joiner.on("', '").join(JS_ARGS_TO_ENUM.keySet());
        throw new IllegalArgumentException(
            "First argument to formatNum must be constant, and one of: '" + validKeys + "'.");
      }
  
      return numberFormatType;
    }