JSFiddle - React, Tailwind, and code Playground

HTML

<div ng-app="myDirective" ng-controller="x">
    <input type="text" ng-model="test" ng-change="updateComplexString()">
    <input type="text" ng-model="test2" ng-change="updateComplexString()">
    <input type="hidden" ng-model="MyComplexString" custom-validation />
</div>

JavaScript

angular.module('myDirective', [])
    .directive('customValidation', function () {
    return {
        restrict: 'A',
        link: function (scope, element, attrs) {
            scope.$watch(attrs.ngModel, function (v) {
                console.log('value changed, new value is: ' + v);
            });
        }
    };
});

function x($scope) {
    $scope.test = "";
    $scope.test2 = "";
    $scope.updateComplexString = function() {
        $scope.MyComplexString = $scope.test + $scope.test2;
    }
}