code: function parseSymbol () {
    if (tokenType === TOKENTYPE.SYMBOL) {
      if (token === 'true') {
        getToken();
        return true;
      }
      if (token === 'false') {
        getToken();
        return false;
      }
      if (token === 'null') {
        getToken();
        return null;
      }
  
      throw createSyntaxError('Unknown symbol "' + token + '"');
    }
  
    return parseEnd();
  }
docs: Parse constants true, false, null
  @return {boolean | null}