[WorldMaker]

JS (and TS thereby) allows arbitrary unicode in identifiers (variable names, class names, [TS] type names). The lambda symbol is just acting as the name of a type parameter instead of something more ASCII usual like `T` or `F`.

There's a lot of interesting discussions around it as some math libraries like to use math symbols and greek letters and sometimes even emoji as variable/class names in JS and other developers have a gut reaction that they do not like it.