JCB is quite common in Japan, just like UnionPay in China, so it is very wise to take a Japanese yen JCB to Japan, because you can keep accounts directly in Japanese yen, and there will be no currency conversion fee.
If JCB dollars are used in Japan, currency exchange procedures will be carried out, that is, yen will be converted into dollars. If so, it is best to use VISA or mastercard, because the exchange rates of VISA and Master are generally better than JCB.