How to unit test Angular controller with $scope.$on? - unit-testing

Question

I have a controller with an event listener in my Angular app, defined as follows.
angular.module('test').controller('TestCtrl', function($rootScope, $scope, testService) {
[...]
$scope.$on("myEvent", function(args, value) {
testService.doStuff(value);
});
});
This works perfectly in the app itself. However, when I try to unit test the functionality of the controller (using Jasmine and Karma), each test method throws the following error:
TypeError: $scope.$on is not a function in [...]/testController.js
I create the controller in my test methods as follows.
it('does stuff', inject(function($rootScope, $controller, testService) {
var scope = $rootScope.$new;
$controller('TestCtrl', {
$scope : scope
});
[...]
}));
I can get around the problem by changing my controller to use rootScope instead:
$rootScope.$on(...)
but then of course the app doesn't work anymore as expected. I can also get rid of the error by introducing the $on method in my test code:
var scope = $rootScope.$new;
scope.$on = function() {};
but mocking the listener like this kind of defeats the purpose of testing my real code.
Why doesn't the test code find the $on method of the scope? (But finds it on rootScope, still...)
There must be something fundamental that I'm missing here but I haven't been able to figure it out even after lots of googling and reading the Angular source code.

Answer 1

$rootScope.$new is a function. You need to invoke it ($rootScope.$new()):
it('does stuff', inject(function($rootScope, $controller, testService) {
var scope = $rootScope.$new();
$controller('TestCtrl', {
$scope : scope
});
[...]
}));
Example plunk: http://plnkr.co/edit/t1x62msmOQDkNItGzcp9?p=preview