How to Test for Null Value

How to Test for Null Value

Hi 

I have 4 snumber boxes in a form. I dont like the default value being 0 as in mobile form i have to delete it each time as  i type in correct number. 

However I have the following forula in code where i have a x b + c + d. Works great as long as i put a number in each. But sometimes i dont need a number in each box and it throws an error saying for example box c is null value. 

How do i test for a null value. I have tried following but not wokring: 

Any help would be much appreciated. 

  2. TLong = input.Trays_Long;
  3. TDeep = input.Trays_Deep;
  4. TStart = input.No__of_Trays_Start;
  5. TEnd = input.Trays_At_End;

  7. if(TStart=isnull(input.No__of_Trays_Start);
  8. {
  9. alert "Null value checker";
  10. }



  14. input.Total_Trays = TLong * TDeep + TStart + TEnd;