I find that jealousy can be both really useful and really useless. I think that some people use it as an excuse to control others, (which is what I’ve had experience with), but at the same time, without it, most relationships probably wouldn’t work out. As for platonic jealousy, I think it can be understandable, especially if someone you’ve been friends for for ages suddenly finds a new friend. The best way to deal with either type though, is definitely just straight up talking to them. Otherwise it’ll eat away at you and they’ll have no idea, when they could probably reassure you and make you feel a lot better.