What is the lifecycle of a service in Angular 5

Services only live inside the scope of their providers, so in the scope of a module or a single component. They are instantiated when injected for the first time and keept alive as long as the provider exists.

As services are normal classes, angulars lifecycle hooks do not apply to them.

Service can have 2 scopes.

If service is declared on your module, you have same instance shared for all, this means service will be constructed when the first component/directive/service/Pipe who needs it will be created. Then it will be destroyed when Module itself will be destroyed (most of the time when page is unloaded)

if the service is declared on Component/Directive/Pipe, then 1 instance will be created each time when Component/Directive/Pipe will be created and destroyed when related Component/Directive/Pipe will be destroyed.

You can see it in action

Code testing : 2 services are made for showing when they are created/destroyed.

@NgModule({
  providers: [GlobalService] // This means lifeCycle is related to the Module, and only one instance is created for the whole module. It will be created only when the first element who needs it will be created.
})
export class AppModule { }

Second service is a local component service and will be create for each hello-component instance created, and will be destroyed just before hello-component will be destroyed.

@Injectable()
export class LocalService implements OnDestroy{
  constructor() {
    console.log('localService is constructed');
  }

  ngOnDestroy() {
    console.log('localService is destroyed');
  }
}

@Component({
  selector: 'hello',
  template: `<h1>Hello {{name}}!</h1>`,
  styles: [`h1 { font-family: Lato; }`],
  providers: [LocalService]
})
export class HelloComponent implements OnInit, OnDestroy {
  @Input() name: string;

  constructor(private localService: LocalService, private globalService: GlobalService) {}

  ngOnInit(){
    console.log('hello component initialized');
  }

  ngOnDestroy() {
    console.log('hello component destroyed');
  }
}

As you can see, Service in angular can have OnDestroy life cycle hook.

OnDestroy is applicable for services as addressed in official document: https://angular.io/api/core/OnDestroy

Quote:

A lifecycle hook that is called when a directive, pipe, or service is destroyed. Use for any custom cleanup that needs to occur when the instance is destroyed.