I believe the doc string of put! on the ClojureScript side is wrong. The "Will throw if closed." part. https://github.com/clojure/core.async/blob/df5312ae038df009e1f067115296a67c81f0305c/src/main/clojure/cljs/core/async.cljs#L126

(let [ch (a/chan 1)]
 (a/close! ch)
 (a/put! ch 42))
=> false