Client Script - onSave event doesn't take Mandatory fields into Consideration

Client Script - onSave event doesn't take Mandatory fields into Consideration

I have several mandatory fields setup in a module, and I am using the onSave Event in Client Script to invoke a custom connection 

Now the problem is, the onSave event is executed even if there's an error such as Missing Mandatory Fields or Validation Rules errors 

Doesn't it make sense for onSave event to be executed after checking all the Mandatory fields and validation rules 

Is there a way I can do that?

Like can I check if there's any Mandatory field errors or validation rule error before invoking the connection in onSave