Even if any given developer doesn't use it as a primary development language, then as something they need a little bit of to do something in a browser, so I'd bet most programmers have at least seen it. And even if you've never seen Javascript before, it's superficially similar to C++/Java/C# so you can kind of make sense of it in simple examples like this.

It's a sufficiently malleable language where you can express ideas from radically different languages with differing philosophies (FP, OO, immutability etc)