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 :)