checkcljs.clj

(cljs.core.typed/cf
(defn ^{:ann '[(U nil (cljs.core/ISeqable Any)) Any -> int]}
  index-of [xs x]
  (let [len (count xs)]
    (cljs.core.typed/loop> [i :- int, 0]
      (if (< i len)
        (if (= (nth xs i) x)
          i
          (recur (inc i)))
        -1))))))
;=> Any