Use the Angular scope.$watch function to observe changes to a scope variable

angular-scope-$watch

/*
 * Observe the "recentTimesheets" scope variable and calculate a derived value when
 * it changes.
 * 
 * The last parameter (true, in this case) instructs the watch to use object equality.
 * If we set this to false or omitted it, the watch expression would use reference
 * equality instead. This can make a huge difference!  For instance, if you are using
 * an Angular $resource to set the watched variable, the $resource.xxx() methods
 * return an empty object immediately, then populate it when the XHR returns.
 *
 * Do your tests not trigger the watch expression?  That's because we need to invoke
 * scope.$apply().  This usually gets triggered automatically, but not in tests.
 *
 * See http://docs.angularjs.org/api/ng.$rootScope.Scope
 */

$scope.$watch("recentTimesheets", function () {
    $scope.creatableTimesheetDates = creatableTimesheetDates();
}, true);

/*
 * Do your tests not trigger the $watch expression?  That's because we need to invoke
 * scope.$apply().  This usually gets triggered automatically, but not in tests.
 */
it("should update scope.creatableTimesheetDates when scope.recentTimesheets changes", function () {
    scope.creatableTimesheetDates = "before";
    scope.recentTimesheets = [];
    expect(scope.creatableTimesheetDates).toBe("before"); // haven't called $apply yet
    scope.$apply();
    expect(scope.creatableTimesheetDates).not.toBe("before");
};