Most operators include the index of the current element in the selector function For example lter(predicate: function(value: T, **index**: number) and mergeMap(project: function(value: T, **?index**: number).

delayWhen however is missing the index:

public delayWhen(delayDurationSelector: function(value: T): Observable, subscriptionDelay: Observable): Observable

However, it would be especially useful if delayDurationSelector included the index. That would make it easy to create an incremental delay, for example when delayWhen is combined with retryWhen.