The reason Final Form does this is so that pristine will be true if you start with an uninitialized form field (i.e. value === undefined), type into it (pristine is now false), and then empty the form field. In this case, pristine should return to true, but the value that the HTML DOM gives for that input is ''. If Final Form did not treat '' and undefined as the same, any field that was ever typed in would forever be dirty, no matter what the user did.

There are many strings which I need to convert if not undefined. I could do as follows: { date1: date1 ? formatISODate(date1) : undefined, date2: date2 ? formatISODate(date2) : undefined ...