The History of j
The letter j is rather different to those discussed so far, as it did not exist in Latin and, of course, a great deal of the medieval literate tradition is in Latin.
So basically, it is probably incorrect to say that j is exactly the same as i in both ancient and medieval Latin. More correctly, there was simply no such letter as j.
The letter j is not particularly common in English. In late medieval writings it might take the form of a minuscule i or a capital I, in which case words beginning with i were often also adorned with a capital.
It came to represent different sounds in different vernacular languages.
The History of the Letter J & Why J-esus was NOT his Name