Some Operator of RxJs

take(n): emits N values before stopping the observable.
takeWhile(predicate): tests the emitted values against a predicate, if it returns `false`, it will complete.
first(): emits the first value and completes.
first(predicate): checks each value against a predicate function, if it returns `true`, the emits that value and completes.
takeUntil(notifier: Observable): Observable - Emit values until provided observable emits.