Reservation of seats

Hello
Upon purchase of JR pass, can we reserve our seats online for Narita Express and the shinkansens despite we have not reach Japan?
Appreciate advice, please.
Thank you.

Only if you buy the online version. Not the exchange voucher version.

Making seat reservation using the JR Pass can only be done at a JR ticket office. Any ticket office can make you a reservation for any JR train, but you're going to have to be in Japan already. Once in Japan, you can make advance reservations for your other journeys as well.

Out of interest, why do you think you need to do this in advance?

In my experience, reservations where available for the N'EX on the day of arrival even in peak tourist weeks, in pre-pandemic times.

Read this page carefully and youfll find your answer, which is gyes, ifch

https://www.japan-guide.com/e/e2361.html

Anyway I wouldnft reserve for NEX as you canft plan which one youfll be able to catch. For Shinkansen, I personally am mostly going by unreserved seats, but I do get it that people want to reserve if traveling with luggage (you need the seat in the last row) or in groups.

You shouldn't reserve a seat on the N'Ex until you're ready to board because you don't know if your flight will arrive late and/or the procedure times at Narita are not known.