How to check if a variable has been assigned a value?

We can check for null to see if the variable has been assigned a value:

        var aString = null;

        if(aString == null){

           document.writeln("Not assigned");

        }

        aString = "some value";

        if(aString == null){

           document.writeln("Not assigned");

        }else{

           document.writeln("Assigned");

        }