What kind of space junk!?
Micrometeorites.
But there is no need in panic. Though Hubble has sustained little injuries over its lifespan, none of them have affected the telescope’s ability to do science or take stunning images and send them back on Earth.
The same thing is about JWST. Its mirrors are protected from such kind of influence. It means that despite of meeting micrometeorites JWST will be able to focus and take pictures.
Moreover, unlike Hubble JWST has been sent far away from Earth's orbit, which means, that the probability of its meeting the satellite or orbital space debris is quiet low.