anonymous function doesn't always return a value
===

I've got JavaScript code like this =>

    (function(){
        document.getElementById("element").onclick = function(){
            var r = confirm("Are you sure ?");
            if (r){
                return true;
            } else {
                return false;
            }
        }
    })();

this script works, but just gives me notification about **Strict Warning** that **anonymous function doesn't always return a value**

I'm interested in what that means, how can I prevent this and will it provoke any problem ? Please any ideas ? thanks :)