code: function computeValue(valueStr) {
      var resultStr = '';
      // length data unit
      var unit = ['%', 'in', 'cm', 'mm', 'em', 'ex', 'pt', 'pc', 'px', 'rem', 'vh', 'vw', 'vmin', 'vmax', 'ch'];
      var unitStr = unit.join('|');
      var unitRE = new RegExp(unitStr, 'g');
      var tempStr = valueStr.match(unitRE)[0];
      if(tempStr) {
          valueStr = valueStr.replace(tempStr, '');
          valueStr = eval(valueStr);
          resultStr = valueStr + tempStr;
      }
      return resultStr;
  }
docs: [computeValue description]
  @param  {[String]} valueStr [css value to be calculated]
  @return {[String]}
  
  '640 / 640 * 10 rem' => '10rem'