[jw-]

"So, is JavaScript a truly functional programming language? The short answer is no. Without support for tail-call optimization, pattern matching, immutable data struc- tures, and other fundamental elements of functional programming, JavaScript is not what is traditionally considered a truly functional language."

Time again I'm always surprised by how people over look type systems when talking about functional languages. Really, much of that list is not fundamental to FP, but always giving me an Int when you promise one matters.

[seanmcdirmid]

Don't tell the Lisp people that. Plenty about functional programming has been explored without static typing.

[Touche]

People don't overlook it, it's not a fundamental property of functional programming.

[smallnamespace]

But by that metric, you could call assembly a functional programming language and not be incorrect either.

There's a core set of features that FP languages implement so that people can be productive, and rich and expressive types certainly seems a key component.