AngularJS - toggle button with ng-class

<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Example - example-number-input-directive-production</title>
  

  <script src="//ajax.googleapis.com/ajax/libs/angularjs/1.3.0/angular.min.js"></script>
  <style type="text/css">
    input[type="button"]{ 
      background-color: green;
    }
      
    input[type="button"].checked{ 
      background-color:orange;
      
    }
  </style> 

  
</head>
<body ng-app="numberExample">
   <script>
   angular.module('numberExample', [])
     .controller('ExampleController', ['$scope', function($scope) {
       $scope.response = {value : true};
       $scope.toggleResponseValue = function(response){
          if(!response.value){
            response.value = false; //Assume it's not clicked if there's no existing value
          }
          response.value = !response.value;
        };
        $scope.buttonState = function(response){
          if(response){
            if(response.value == true){
              console.log('returning checked');
              
              return ['checked'];
            }
          }
        };
     }]);
 </script>
 <form name="myForm" ng-controller="ExampleController">
   <input type="button" 
   value="click me to end the world" 
   ng-click="toggleResponseValue(response)" 
   ng-class="{'checked':response.value}">
   <!-- ng-class="buttonState(response)" -->
   <tt>response = {{response}}</tt><br/>
  </form>
</body>
</html>

  var value = element(by.binding('value'));
  var valid = element(by.binding('myForm.input.$valid'));
  var input = element(by.model('value'));

  it('should initialize to model', function() {
    expect(value.getText()).toContain('12');
    expect(valid.getText()).toContain('true');
  });

  it('should be invalid if empty', function() {
    input.clear();
    input.sendKeys('');
    expect(value.getText()).toEqual('value =');
    expect(valid.getText()).toContain('false');
  });

  it('should be invalid if over max', function() {
    input.clear();
    input.sendKeys('123');
    expect(value.getText()).toEqual('value =');
    expect(valid.getText()).toContain('false');
  });