I am trying to write a validation block inside my JS to check if the user has selected a value or not and pop up a message if the value is not selected.

function validate(form) {

    var success = true;

    var message = ;



    if (form.dropdown.selectedIndex ==  0 )  {  

        form.save.disabled=true;



        if (0 < message.length) {

            message += n; 

        }

        message += dropdown  value should be selected.;

    }



    if (0 < message.length) {

        alert(message);

        success = false;

    }



    return success;

}

When I click on Save button I call this function. I don't see errors in the logs.
Can anyone tell me what am I doing wrongly here? Or can you guide me what is the correct method to validate if the user has selected a value before allowing to save?

Thanks!