For those that are confused by the name change, Jack is an old nickname for John. It was most popular in the 17th century and has been on the decline since, so I don’t think many people today use it that way anymore. Or any people younger than 70, anyway.
Of a similar origin, there’s Jim for James, Dick for Richard (giggity), Harry for Henry, and Sally for Sarah, among others. I think some of these are more popular than others, but it seems to me that they are mostly out of use in general, though some of the nicknames live on as normal names.