Best way to "flatten" an array inside an RxJS Observable

Actually if you need it inside the stream just use this:

.flatMap(items => of(...items))

Angular 6 note.

If you are using as a pipeable operator, do is known as tap!

https://www.learnrxjs.io/operators/utility/do.html Here is an example.

// RxJS 
import { tap, map, of, mergeMap} from 'rxjs/operators';

backendSource
  .pipe(
   tap(items => console.log(items)),
   mergeMap(item => item ),
   map(item => console.log(item.property))
  );

You can use concatAll() or mergeAll() without any parameter.

dataFromBackend.pipe(
  tap(items => console.log(items)),
  mergeAll(), // or concatAll()
)

This (including mergeMap) works only in RxJS 5 because it treats Observables, arrays, array-like objects, Promises, etc. the same way.

Eventually you could do also:

mergeMap(val => from(val).pipe(
  tap(item => console.log(item)),
  map(item => item.name),
)),
toArray(),

Jan 2019: Updated for RxJS 6

If it is a synchronous operation, I would suggest to use javascript's Array.map instead, it even should save you some performance:

const dataFromBackend = Rx.Observable.of([
  { name: 'item1', active: true },
  { name: 'item2', active: false },
  { name: 'item3', active: true }
]);

dataFromBackend
  .map(items => items.map(item => item.name))
  .subscribe();