Rx.Observable.prototype.indexOf(searchElement, fromIndex)
Returns the first index at which a given element can be found in the observable sequence, or -1 if it is not present.
Arguments
searchElement(Any): An element to locate in the array.fromIndex(Number): The index to start the search. If not specified, defaults to 0.
Returns
(Observable): An observable sequence containing the first index at which a given element can be found in the observable sequence, or -1 if it is not present.