using variable

using variable

I have the following code:
  1. var weight = "";
                    weight = $("#selectWeight").val();
                    if(weight == '')
                    {
                        alert('you must select a value first!');
                    }

If weight has no value, I do not get a alert box.  If I add an alert after getting the weight value, then I get the alert box.  Is there something about using/initializing variables that I'm missing?