Q:

Angular directive change ngmodel value from null to undefined breaks validation

I have created a plnkr to describe my problem: Link to plnkr
Problem description:
I have a number field, the value from which is written to the model. First I implemented this feature like the first input. The problem with this implementation is that if I input something and then delete, I have the following model:
{"firstNumber":null,"secondNumber":64}

For me unfortunately this representation is not acceptable, I need the following result:
{"secondNumber":64}

For that I taken the directive and implemented the second field. Now I receive the correct output, but when I remove the value, the form becomes invalid.
Also I added third and fourth input to demonstrate that the directive also breaks the required validation.
So, the question is:
How can I improve the input field not to have the model 
{"firstNumber":null,"secondNumber":64}

but
{"secondNumber":64}

and do not broke the validation mechanism of Angular forms.
For reference:
I have the following HTML:
<body ng-controller="MainCtrl as vm">
<h1>Validating input inside ng-repeat with Angular 1.3</h1>

<form name="vm.myForm" novalidate>
  <input type="number" ng-model="vm.fields.firstNumber" name="firstNumber">
  <input type="number" ng-model="vm.fields.secondNumber" name="secondNumber" null-to-undefined>
  <br>
  <input type="number" ng-model="vm.fields.thirdNumber" name="thirdNumber" ng-required = "true">
  <input type="number" ng-model="vm.fields.fourthNumber" name="fourthNumber" null-to-undefined ng-required="true">
</form>
</body>

And following controller and directive code:
var app = angular.module('plunker', []);

app.controller('MainCtrl', function($scope) {
  var vm = this;

  vm.fields = {};
  vm.fields.firstNumber = 12;
  vm.fields.secondNumber = 24;
  vm.fields.thirdNumber = 64;
  vm.fields.fourthNumber = 128;

});

app.directive('nullToUndefined', function($timeout) {
  return {
    restrict: 'A',
    require: 'ngModel',
    link: function(scope, elem, attrs, ctrl) {
      ctrl.$parsers.push(function(viewValue, modelValue) {
        if (viewValue === null) {
          $timeout(function() {
            //ctrl.$setValidity('number', true);
          });
          return undefined;
        }
        return viewValue;
      });
    }
  };
});

p.s. I cut the code as much as possible to reproduce the problem from my main project. I would appreciate any solution, but it would be very cool, if somebody can suggest the solution, which:
1. Shows good performance
2. Behaves in angular way.

A:

You should probably focus on other solution, instead of writing directive.
You have object with props (firstNumber, secondNumber, ...).
You need to validate property with Angular (is null ? is correct number ?).
Then, you need to filter the object props with non-null values, without mutation of your model : your input is bind to model property, if you destroy a property, Angular can't be able to validate and fail.
We can do it :

Select all keys of a.fields
Iterate all keys, filters
We got all non-null keys
Reduce non-null keys to a new object, without mutation of "a"

const a = {
  fields: {
    a: 1,
    b: 2,
    c: null,
  },
};

// After form validation
const newA = Object
  .keys(a.fields)
  .filter(k => a.fields[k] !== null)
  .reduce(
    (accumulator, k) => 
      Object.assign(
        accumulator,
        { 
          fields: Object.assign(accumulator.fields, { [k]: a.fields[k] }) 
        }
      ),
    { fields: {} }
  );

And if you need to filter undefined + null, just do little check on filter :
  .filter(k => a.fields[k] != null)

Edit : fix code.

