How to check the dropdown selected value is null or not.

Iam using the following code ,

if(input.dropdown!=null && input.dropdown!=""),But this is not working for me.

EX;if(input.dropdown=="Some value") 

        show "Some value"

    else if(input.dropdown=="--select--")

        hide "--Select--";//i need to hide the "Some value" but that is not happening.

 

Please suggest any solution.