Unit Testing with Angular >= 14 without TestBed and Inject Function

Since the introduction of Angular 14 by the Angular Team, a new 'inject' function has been implemented. This function enables the use of services or other dependencies in a component without the need of a constructor. However, this requirement leads to the use of the TestBed in testing the service, which can often be slightly frustrating for efficient unit tests.

However, there is a workaround for this, by importing everything from the @angular/core, and then the jest in combination with spyOn can be used on the inject function. Subsequently, you can simply verify the providerToken and provide the corresponding mock or service as a return value. Here's an example:

1// import * as core from '@angular/core';
2...
3jest.spyOn(core, 'inject').mockImplementation((providerToken => {
4    if (providerToken === ApiService) {
5        return mockApiService;
6    }
7}));
8...

If you want more details on this, Rainer Hahnekamp has released a very good video on the topic.