Dependency Injection (DI) is utilized heavily in Angular, primarily for providing instances of either primitive values or class instances.

DI is a complex topic that requires a separate discussion which would be out of the scope of this handbook. We recommend checking out the following resources:

• The official DI guide which explains many details
• This repo for some examples
• This useful infographic that gives a good graphical overview of the whole DI mechanism

As of Angular 6, make sure to use { providedIn: 'root' } for singletons whenever possible. Alongside this, prefer to inject dependencies using the inject function over constructor.

// OK, but not preferred
constructor(private readonly userService: UserService){}

// Preferred way
private readonly userService = inject(UserService);

DI is very useful for testing purposes, as shown in a later chapter.