It's not a javascript compiler as Clojure is a hosted language.

Nbb isn't a compiler, but a Clojure interpreter. Clojure is a hosted language, but can be compiled or interpreted. The ClojureScript compiler compiles Clojure to JavaScript, but nbb only interprets Clojure.

Can anyone explain what a hosted language is?

Basically, you take a programming language and make it work on a platform that meant to be programmed using a different PL. Clojure is hosted by design - it's not Java, but can be used to program for JVM. It ain't Javascript, yet can be used to target nodejs and browser; not an [official] CLR language, but you can write .Net programs. You can use Clojure to make Flutter apps with ClojureDart. You can integrate Python into Clojure with libpython-clj. Or write Clojure to target Erlang/OTP; or Rust; or R; There's even a clojure-like language for Lua - Fennel.

There's something about Clojure people like so much, they want it to work atop any platform.

https://github.com/Tensegritics/ClojureDart

https://github.com/clj-python/libpython-clj

https://github.com/clojerl/clojerl

https://github.com/clojure-rs/ClojureRS

https://github.com/scicloj/clojisr

https://fennel-lang.org

Compiles/transpiles into code that runs on a "host" language execution environment (nodejs, JVM, CLR) instead of it's own VM implementation, or native code.

clojurescript absolutely compiles to javascript, as per the official docs:

https://clojure.org/about/clojurescript

But that's not what nbb is.