ngrx - chain two store.select slices

Try this:

this.todoListGroup$ = this.ngrx.select(fromRoot.getTodos)
.flatMap((todos: Todo[]) => {
    console.log(todos)
    this.todos = todos
    this.ngrx.select(fromRoot.getLastChangedTodo);
})
.take(1)
.map(lastTodo =>{{
    console.log(lastTodo)
    doSomething(this.todos, lastTodo)
})

This is an approach for RxJs v6+ using the combineLatest operator.

First import the operators and Observable function like this:

import { Observable, combineLatest } from 'rxjs';
import { tap } from 'rxjs/operators';

Then you can use them like this:

combineLatest([
  this.store.pipe(select('users', 'userList')),
  this.store.pipe(select('users', 'accountsList')),
]).pipe(tap(([userList, accountsList]) => console.log(userList, accountsList)))

Would something like this work:

this.todoListGroup$ =
    Observable.combineLatest(
        this.ngrx.select(fromRoot.getTodos), 
        this.ngrx.select(fromRoot.getLastChangedTodo)
    )
    .do(([todos, lastToDo]) => console.log(todos, lastToDo));

The do would execute each time either one of getTodos or getLastChangedTodo is updated and would take the latest known values from each of them at the time of the update. The caveat here is the order of when each of those updates are fired may not always be the same. So, if you wanted more of a chained (or cascaded) update then you could do this:

this.todoListGroup$ =
    this.ngrx.select(fromRoot.getTodos)
    .withLatestFrom(this.ngrx.select(fromRoot.getLastChangedTodo))
    .do(([todos, lastToDo]) => console.log(todos, lastToDo));

That will execute each time getToDos is updated and would take the latest value from getLastChangedTodo. Hence the chained (or cascaded) updated idiom.

edit for rxjs 5+ syntax:

this.todoListGroup$ =
    combineLatest(
        this.ngrx.select(fromRoot.getTodos), 
        this.ngrx.select(fromRoot.getLastChangedTodo)
    )
    .pipe(tap(([todos, lastToDo]) => console.log(todos, lastToDo)));


this.todoListGroup$ =
    this.ngrx.select(fromRoot.getTodos).pipe(
      withLatestFrom(this.ngrx.select(fromRoot.getLastChangedTodo)),
      tap(([todos, lastToDo]) => console.log(todos, lastToDo))
    );