In Angular, we can mock out services using $provide. Let's look at a simple example.

var app = angular.module('my-site', []);

app.factory('locale', function ($window) {
  return $window.location.pathname.split('/')[1];
});

app.factory('currency', function (locale) {
  switch (locale) {
    case 'en-us':
      return 'usd';
    case 'de-de':
      return 'euro';
  }
});

Here we have two services using the .factory() API in our module. The first service is locale and it reads the locale from the URL. The locale would be something like en-us, de-de, etc. The second service is currency and it returns the currency based on the user's locale. currency is dependent on the locale service.

When we unit test currency, we want to mock out locale because there is no URL to read the locale from in the test environment. We want to test currency in isolation and see how currency behaves for the different values that locale could be. Here is a corresponding unit test:

describe('currency', function () {
  beforeEach(module('my-site'));
  beforeEach(
    module(function ($provide) {
      $provide.value('locale', 'en-us');
    })
  );

  it('should be usd if the locale is en-us', inject(function (currency) {
    expect(currency).toEqual('usd');
  }));
});

In this example $provide.value() is used to mock out the value of the locale service. Whenever a service asks for locale from Angular, Angular will instead inject this mock value instead of trying to get the locale from the URL as defined in the original locale service.

$provide has methods for mocking out other Angular services including factories, providers, services with the .service() API, values, and constants. For example, the above test could also be written using $provide.factory():

describe('currency', function () {
  beforeEach(module('my-site'));
  beforeEach(
    module(function ($provide) {
      $provide.factory('locale', function () {
        return 'en-us';
      });
    })
  );

  it('should be usd if the locale is en-us', inject(function (currency) {
    expect(currency).toEqual('usd');
  }));
});

$provide is a useful service in unit testing to mock out service dependencies and isolate the service under test.