Angular Services

@Injectable({ # service decorator
    providedIn: 'root' # singleton service
})
export class DataService {
    getData() {
        return [1, 2, 3];
    }
}

ng generate service data # CLI command
ng g s data # shorthand

export class AppComponent {
    constructor(private dataService: DataService) { # inject service
        this.data = dataService.getData();
    }
}

import { HttpClient } from '@angular/common/http';
constructor(private http: HttpClient) { }
this.http.get('api/data').subscribe(data => { });

providedIn: 'root' # app-wide singleton
providedIn: 'any' # one instance per injector
providers: [DataService] # in component/module