Hashing for custom types such that they are properly detected as duplicates within ClojureScript's PersistentHashSets.

ihash.cljs

(deftype TestType [key]
  IHash
  (-hash [this]
    (hash key)))

(def a (TestType. [1 2 3]))
(def b (TestType. [1 2 3]))

;; the resulting PersistentHashSet should have one item, but has two:
(.log js/console (pr-str (conj #{} a b)))
;; → #{#<[object Object]> #<[object Object]>}

(.log js/console (== (hash a) (hash b)))
;; → true

I see, PersistentHashSet uses IEquiv's -equiv to test for duplicates.