map-operators-get-all-03.ts

groupBy(
  /**
   * Select groups by their `bookId`
   */
  action => action.bookId,
  /**
   * Map each action to the `bookId` for convenience
   */
  action => action.bookId,
  /**
   * The duration selector lets us create an observable that
   * instructs `groupBy` when to dispose of a group. It will
   * be disposed when either the returned observable emits a
   * a value or completes
   */
  bookId$ => bookId$.pipe(
    /**
     * The `timeoutWith` operator lets you time out an observable
     * after a set amount of time and then falls back to the
     * provided observable.
     *
     * Here we want to timeout after 15s and then fall back to
     * no behavior
     */
    timeoutWith(15000, Observable.empty()),
    /**
     * When telling `groupBy` when to end we don't want it
     * to dispose of the group when the group has a new action
     * so we simply have it ignore all actions, instead waiting
     * to dispose the group when it times out
     */
    ignoreElements(),
  ),
)