password-check directive in angularjs

I use the following directive because I want to re-validate both input field regardless of whether value 1 or value 2 was changed:

directive:

'use strict';

angular.module('myApp').directive('equals', function() {
  return {
    restrict: 'A', // only activate on element attribute
    require: '?ngModel', // get a hold of NgModelController
    link: function(scope, elem, attrs, ngModel) {
      if(!ngModel) return; // do nothing if no ng-model

      // watch own value and re-validate on change
      scope.$watch(attrs.ngModel, function() {
        validate();
      });

      // observe the other value and re-validate on change
      attrs.$observe('equals', function (val) {
        validate();
      });

      var validate = function() {
        // values
        var val1 = ngModel.$viewValue;
        var val2 = attrs.equals;

        // set validity
        ngModel.$setValidity('equals', ! val1 || ! val2 || val1 === val2);
      };
    }
  }
});

usage

<input type="password" ng-model="value1" equals="{{value2}}" required>
<input type="password" ng-model="value2" equals="{{value1}}" required>