jquery ui dialog box need to return value, when user presses button, but not working

javascript is asynchronous.

so you have to use callbacks:

   $("#modal_confirm_yes_no").dialog({
            bgiframe: true,
            autoOpen: false,
            minHeight: 200,
            width: 350,
            modal: true,
            closeOnEscape: false,
            draggable: false,
            resizable: false,
            buttons: {
                    'Yes': function(){
                        $(this).dialog('close');
                        callback(true);
                    },
                    'No': function(){
                        $(this).dialog('close');
                        callback(false);
                    }
                }
        });
    function callback(value){
         //do something
    }