import { Observable, from } from 'rxjs';
import { distinctUntilKeyChanged } from 'rxjs/operators';

const observable$: Observable<string> = from(["hello", "lemon", "car", "bike", "apple", "knife", "night"]);

observable$
  .pipe(
    distinctUntilKeyChanged('length')
  )
  .subscribe(x => console.log(x));