How to use the graphql-language-service-parser.opt function in graphql-language-service-parser

// JSON Number.
  Number: /^-?(?:0|(?:[1-9][0-9]*))(?:\.[0-9]*)?(?:[eE][+-]?[0-9]+)?/,

  // JSON String.
  String: /^"(?:[^"\\]|\\(?:"|\/|\\|b|f|n|r|t|u[0-9a-fA-F]{4}))*"?/,

  // JSON literal keywords.
  Keyword: /^true|false|null/,
};

/**
 * The parser rules for JSON.
 */
const ParseRules = {
  Document: [p('{'), list('Variable', opt(p(','))), p('}')],
  Variable: [namedKey('variable'), p(':'), 'Value'],
  Value(token) {
    switch (token.kind) {
      case 'Number':
        return 'NumberValue';
      case 'String':
        return 'StringValue';
      case 'Punctuation':
        switch (token.value) {
          case '[':
            return 'ListValue';
          case '{':
            return 'ObjectValue';
        }
        return null;
      case 'Keyword':