Angular Services and Dependency Injection Quiz Quiz

Challenge your understanding of Angular services and dependency injection principles with these focused questions covering provider configuration, service lifecycles, and best practices. This quiz is designed for developers seeking to reinforce their knowledge of Angular's architecture and service management capabilities.

1. Understanding Singleton Services

   When you provide a service in the root injector using 'providedIn: root', what is the primary effect on the service's lifecycle throughout your Angular application?

   1. A single instance of the service is shared across the entire application
   2. The service can only be used in the root module
   3. The service becomes available only in child modules
   4. A new instance of the service is created for each component

   Explanation: Providing a service in the root injector ensures a single, shared instance for the whole application, implementing the singleton pattern. If you created a new instance per component, you would not use providedIn: root. Limiting usage to the root module is incorrect, as the service is globally accessible. The service is not restricted to child modules; instead, it is available everywhere unless provided differently.

2. Registering Services in Angular

   Which configuration is used to make a service available only to the components in a particular Angular module and not globally?

   1. Including the service in the module's 'providers' array
   2. Using 'providedIn: root' in the service's decorator
   3. Declaring the service in the 'declarations' array
   4. Adding the service to the component's 'imports' array

   Explanation: Including the service in a module's 'providers' array restricts its availability and lifecycle to that specific module and its components. 'providedIn: root' makes it globally available, not module-scoped. Adding a service to the 'imports' or 'declarations' arrays is incorrect; imports are for modules, and declarations are for components, directives, and pipes, not services.

3. Dependency Injection in Component Constructors

   How does Angular inject a service into a component, as seen in an example where a LoggerService is passed into a component's constructor?

   1. By adding the service to the component's selector
   2. By calling a static method from the service in the component's template
   3. By passing the service as a parameter in the component's constructor
   4. By manually instantiating the service inside the component using 'new'

   Explanation: Angular injects services by passing them as parameters in component constructors, leveraging the framework's dependency injection system. Manually creating a service instance with 'new' bypasses the DI system, which is not recommended. Static methods in the template and selectors do not handle service injection. Only the constructor parameter option aligns with best practices and automatic injection.

4. Default Injector Hierarchy

   If a service is provided only in a specific Angular component's 'providers' metadata, what will happen when that service is injected into both the component and its child components?

   1. The service cannot be injected into child components
   2. Each component instance receives its own unique service instance
   3. The service becomes available only to imported modules
   4. A single service instance is shared across all components in the application

   Explanation: When a service is listed in a component's 'providers', Angular creates a unique instance for that component and its view descendants. This is different from using the root injector, which shares a single instance app-wide. The service is not shared globally nor restricted to imported modules, and it does remain available to child components, contrary to stating it cannot be injected there.

5. Avoiding Common Mistakes in Service Usage

   Which of the following is considered a best practice when using services with dependency injection in Angular applications?

   1. Defining services inside component classes for encapsulation
   2. Injecting services through constructors rather than creating them with 'new'
   3. Using services exclusively in templates without injecting them
   4. Providing the same service in every component's 'providers' property

   Explanation: Injecting services through constructors allows Angular to manage service lifecycles and dependencies efficiently. Using 'new' manually breaks the benefits of dependency injection. Services should not be used only in templates or defined inside components, as their purpose is reusability and separation of concerns. Providing the same service in every 'providers' property would create multiple instances, which is generally considered an anti-pattern.