i want to execute javascript on pageunload if var is true

Question

the goal of this script is to only execute on page close when edited (a bool in ASP) is true

here is what the code looks like....

window.onbeforeunload = function (){
     if (<%=edited%>){  
         return 'Are you sure you want to leave?';  }

      };

how would i get this to work?