Set new value with dependent() is not working

Ronald76

hi together,
i try to update a value in database,
if the value in a field "main_menu" is by "1" the field "sub_menu" need the value "0".
I think about a code like this:

editor.dependent( 'main_menu', function ( val ) {
        if (val == '1') {
                 editor.field( 'sub_menu').val("0");
        }
});

whatever i try to set the value doesnt work. maybe i can get help here?

Thanks
Ronny