It turns out that John Travolta started balding a long time ago and hid the whole thing with a hairpiece. But, there are a few occurrences of him in public without his wig on. Here's John Travolta before and after his toupee:
It's clear that John Travolta didn't have a hair transplant because you can see the netting from his wig. Usually the specialist will be able to bleach / blend the netting better than that. The object is to make the netting blend with the skin tone.