Require additonal comments when incident is cancelled

[Patrick Slatter]

Morning! 

 I am trying to make additonal comments required when an incident is cancelled, however i am experiancing some strange behaviour. 

Client script OnChange 

UI type = ALL

Field = State 

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
if (isLoading || newValue === '') {
return;
}
// var state = g_form.getValue('state');

if (newValue == 8 ) {

g_form.setMandatory('comments', true);
g_form.showFieldMsg('comments',"Additional comments are required when cancelling an incident.");

}
}

When this runs it pops up the additonal comments field, however it is not required.

Example of odd behaviour:

If it is in a new state, and i change to cancelled, nothting happens. If it is in a new state, i change to work in progress then change to cancelled (all before saving) then the field does become required.

Any assistance would be greatly appriciated. 

Thanks!

[Swetans]

where to add the above script?