[rafram]

The point is that developers like this purposefully introduce new languages and frameworks that only they (and maybe not even they) fully understand, so they make themselves harder to replace and ensure that they appear ahead of the curve. (Then they get bored and leave anyway.) It's totally valid to introduce new technologies when the problem calls for them, but doing it in a way that's purely detrimental to the rest of the team is very selfish.

[CodingWithJesse]

Exactly my point. Learning stuff is fun. But if you're going to introduce new technologies or programming languages into a system, you need to make sure it suits the skills and experience of the rest of the team. Rewriting the company's admin dashboard in Elm, not letting anyone else work on it, and then quitting - that's just cruel. (Elm's cool, but you know what I mean..)