Unit test - Mock promises · Gabriel Muller - JavaScript and Angular articles

When first writing Javascript and specially when unit testing it, promises can be one of the hardest thing to understand.

Scenario

In this example we would like to mock a simple service returning a promise.

This is a real life example, of an application, calling an employeeService to retrieve the notificationCount.

employeeService.getEmployee().then(function (employee) {
  $scope.unreadNotifications = employee.notificationCount;
});

Usually to mock a function, we would just do:

employeeService.getEmployee = function () {
  return {
    notification: 2
  }
}

However that wouldn’t work here since employeeService.getEmployee returns a promise.

Solution

employeeService.getEmployee = function () {
  var deferred = $q.defer();
  deferred.resolve({
    notificationCount: 2
  });
  return deferred.promise;
};

Analysis

First we create a deferred instance $q.defer()
Then we resolve it passing the object we want {notificationCount: 2}
Finally we return a promise deferred.promise