function parseNumber(state) {
    var numberStr;

    if (state.tokenType === TOKENTYPE.NUMBER) {
      // this is a number
      numberStr = state.token;
      getToken(state);
      return new ConstantNode(numeric(numberStr, config.number));
    }

    return parseParentheses(state);
  }
parse a number
@return {Node} node
@private