Rx.Observable.prototype.expand(selector, [scheduler])
Expands an observable sequence by recursively invoking selector.
Arguments
selector(Function): Selector function to invoke for each produced element, resulting in another sequence to which the selector will be invoked recursively again.[scheduler=Rx.Scheduler.immediate](Scheduler): Scheduler on which to perform the expansion. If not provided, this defaults to the immediate scheduler.
Returns
(Observable): An observable sequence containing a single element determining whether all elements in the source sequence pass the test in the specified predicate.