code: function sum(expression) {
  	
          //Check arguments
          if (!expression)
              throw new Error("Expression is invalid");
  
          //Select only the field(s) matching expression
          var outData = jslinq(this.items)
  			.select(expression)
  			.toList();
  			
  		//Define value of sum
  		var sumValue = 0;
  
          //For each element on outData
          for (var i = 0; i < outData.length; i++) {
  			
  			//Extract current value
  			var currentValue = outData[i];
  
              //If element is not a number
  			if (typeof currentValue != 'number')
  				throw Error("Value '" + currentValue + "' is not a number");
  
              //Sum with total value
              sumValue = sumValue + currentValue;
          }
  
          //Return sum
          return sumValue;
      }
docs: #region "sum" Sum values specified with expression