const items$ = Rx.Observable.of([10, 20, 30], [20,30,40])
  .reduce((acc, curr) => acc.concat(curr));

let itemsArray = [];

items.subscribe(arr => itemsArray = arr);

console.log(itemsArray); // [10, 20, 30, 20, 30, 40]

const itemsArray = Rx.Observable.of([10, 20, 30], [20,30,40])
  .reduce((acc, curr) => acc.concat(curr))
  .collect( /* return a number[]  - not an Observable<number[]> */);


console.log(itemsArray); // [10, 20, 30, 20, 30, 40]