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A reference to the browser's `window` object. While `window` is globally available in JavaScript, it causes testability problems, because it is a global variable. In angular we always refer to it through the `$window` service, so it may be overridden, removed or mocked for testing.

Expressions, like the one defined for the `ngClick` directive in the example below, are evaluated with respect to the current scope. Therefore, there is no risk of inadvertently coding in a dependency on a global value in such an expression.

## Example

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    <script>
      angular.module('windowExample', [])
        .controller('ExampleController', ['$scope', '$window', function($scope, $window) {
          $scope.greeting = 'Hello, World!';
          $scope.doGreeting = function(greeting) {
            $window.alert(greeting);
          };
        }]);
    </script>
    <div ng-controller="ExampleController">
      <input type="text" ng-model="greeting" aria-label="greeting" />
      <button ng-click="doGreeting(greeting)">ALERT</button>
    </div>

    it('should display the greeting in the input box', function() {
     element(by.model('greeting')).sendKeys('Hello, E2E Tests');
     // If we click the button it will block the test runner
     // element(':button').click();
    });