Are butterfly knives legal in New Jersey?

That's sort of a gray area. The answer, I believe, is that they are legal to buy and own but not legal to conceal and carry. One handed opening knives like tactical folders, that open via the use of a thumb stud or thumb hole with a blade that is less than 5" are perfectly legal to own and carry in NJ. However knives that open via a switch or button such as spring assisted folders or switchblades are totally illegal in NJ.

It is possible to buy butterfly knives in Army Navy surplus stores in NJ but such stores do not sell switchblades, so I am assuming it is legal to buy butterfly knives in NJ, otherwise I don't think these stores would risk selling them.