Service-to-service injection in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is service-to-service injection in Angular?

Service-to-service injection is when one Angular service asks for another service to be provided inside it. This lets services share data or functions easily.

beginner

How do you inject a service into another service in Angular?

You add the service you want to inject as a parameter in the constructor of the service that needs it. Angular then provides it automatically.

intermediate

Why use service-to-service injection instead of calling services directly?

It keeps code clean and testable. Angular manages service creation and sharing, so you don’t create multiple copies or tightly couple services.

intermediate

What happens if you inject a service that is not provided in Angular?

Angular throws an error because it cannot find or create the service instance. You must add the service to providers or use @Injectable({providedIn: 'root'}).

advanced

Can service-to-service injection cause circular dependencies? How to avoid it?

Yes, if two services inject each other directly, it causes a circular dependency error. To avoid it, refactor code to remove direct loops or use interfaces and forwardRef.

How do you inject ServiceB into ServiceA in Angular?

A Import ServiceB in ServiceA and create new instance manually

B Call ServiceB methods directly inside ServiceA without injection

C Add ServiceB as a constructor parameter in ServiceA

D Use @Injectable decorator on ServiceA only

You inject ServiceB by adding it as a constructor parameter in ServiceA. Angular then provides the instance automatically.

What decorator must a service have to be injectable in Angular?

A @Injectable

B @Component

C @NgModule

D @Directive

The @Injectable decorator marks a class as available for dependency injection.

What error occurs if a service is injected but not provided?

A No provider for Service error

B ServiceNotFoundError

C NullPointerException

D SyntaxError

Angular throws a 'No provider for Service' error if the service is not registered in providers.

What is a common problem when two services inject each other?

A Memory leak

B Compilation warning

C Slow performance

D Circular dependency error

Injecting two services into each other creates a circular dependency error.

Where can you provide a service to make it available app-wide?

A In the component's providers array

B In the service's @Injectable decorator with providedIn: 'root'

C In the main.ts file only

D In the HTML template

Using providedIn: 'root' in @Injectable makes the service available throughout the app.

Explain how service-to-service injection works in Angular and why it is useful.

Think about how services can talk to each other without creating new instances manually.

Describe how to avoid circular dependencies when injecting services into each other.

Consider how to break the loop so services don’t depend on each other directly.