i am validating email with knockout js and throughing error while if email not valid on the same time i want to invoke a fuction call iam trying to do somthing like that but not working please someone help i know the exact way to do that

 email: ko.observable((ref = REGISTRY.unsavedUser) != null ? ref.email : void 0).extend({
    required: true,
    email: {
      params: true,
      message: 'Please enter a valid emails.',
    },
    validation: {
        validator: testFuc
    },
    focus:true
  }),

and testFuc is as

var testFun = function() {
  alert();

}