How to build Angular 2 apps using Observable Data Services - Pitfalls to avoid

action-method.ts

addTodo(newTodo:Todo):Observable {
    let obs = this.todoBackendService.saveTodo(newTodo);

    obs.subscribe(
            res => {
                this._todos.next(this._todos.getValue().push(newTodo));
            });

    return obs;
}

behaviour-subject.ts

let BehaviourSubject = new BehaviorSubject(initialState);

build-data-service.ts

@Injectable()
export class TodoStore {
    private _todos: BehaviorSubject<List<Todo>> = new BehaviorSubject(List([]));

    public readonly todos: Observable<List<Todo>> = this._todos.asObservable();

    constructor(private todoBackendService: TodoBackendService) {
        this.loadInitialData();
    }
    ...
}

current-value.ts

let currentValue = behaviorSubject.getValue();

duplicate-http-calls.ts

saveTodo(newTodo: Todo) : Observable> {
    var headers = new Headers();
    headers.append('Content-Type', 'application/json; charset=utf-8');

    return this.http.post('/todo', JSON.stringify(newTodo.toJS()),{headers})
        .pipe(
            shareReplay()
        );
}

inject-example.ts

export class App {
    constructor(private todoStore: TodoStore, 
                private uiStateStore: UiStateStore) {
    }
}

modify-service-data.ts

onAddTodo(description) {
    this.todoStore.addTodo(newTodo)
        .subscribe(
            res => {},
            err => {
                this.uiStateStore.endBackendAction();
            }
        );
}

subject-emit.ts

subject.next(newValue);

use-service.html

<ul id="todo-list">
    <li *ngFor="let todo of todoStore.todos | async" >
        ...
    </li>
</ul>

use-subject.ts

let subject = new Subject();
subject.subscribe(value => console.log('Received new subject value: '))

What is this List<> type you use? It's not a typescript built-in type, and it's not in basarat/typescript-collections.
I want to assume it's just an alias for an array, but I don't really like assuming things.

@FacelessPanda. I am going through this and it could be List from the immutable library. Typescript does not mind so far.
import { List } from 'immutable';

Full store example on https://github.com/jhades/angular2-rxjs-observable-data-services/blob/master/src/state/TodoStore.ts

An Angular 7 update would be appreciated...