[Angular 9 Unit testing] Stronger typing for dependency injection in tests

In Angular version 8, TestBed.get was deprecated. In Angular version 9, we see why: TestBed.inject<T> is introduced as a type-safe replacement.

There are two differences between TestBed.get and TestBed.inject<T>:

1. TestBed.get returns any. TestBed.inject<T> returns a value of type T.
2. TestBed.get accepts a token of type any. TestBed.inject<T> accepts a token of type Type<T> | InjectionToken<T> | AbstractType<T>.

The type T in (1) is either a concrete class type, an abstract class type or the value returned by a dependency injection token, as defined by the passed token argument.

(2) is similar to the fact that Injector#get accepted a token of type any in Angular version 2. This signature was deprecated in Angular version 4 and a method signature similar to TestBed.inject was introduced.

This means that in practice, we are able to use for example a string or a number as an injector token. However, this has been a deprecated feature for 3 years and should not be used.

What TestBed.inject means in practice for our tests is that TypeScript now can infer the type of the returned value when resolving dependencies as seen in Listing 1.

// my.service.spec.ts
it('infers dependency types', () => {
  // `service` has inferred type `MyService` in Angular version 9
  const service = TestBed.inject(MyService);
}); 

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