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in ClojureScript by

ES6 Iterables: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols

Use in React: https://github.com/facebook/react/blob/30dac4e78de02fb427ee82013160ae875128d7a2/packages/react/src/ReactElementValidator.js#L195-L204

(defn es6-iterator**
  (if (implements? IIterable coll)
    (let [it (-iterator ^not-native coll)]
      #js{:next (fn []
                  (if ^boolean (.hasNext it)
                    #js{:value (.next it), :done false}
                    #js{:value nil, :done true}))})
    ;; Fallback to naive first/next iterator:
    (ES6Iterator. (seq coll))))

;; Tests can use round trip:
(es6-iterator-seq (es6-iterator (hash-map 0 1 2 3)))

(defn es6-iter-consume
  (while (not (.-done (.next it)))))

(dotimes [_ 3]
  (let [xs (vec (range 3000))
        runs 1000]
    (simple-benchmark []
      (es6-iter-consume (es6-iterator xs)) runs)
    (simple-benchmark []
      (es6-iter-consume (es6-iterator** xs)) runs)))

About a 4x speedup in Chrome. Also note that *much* less garbage is produced.

1 Answer

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Reference: https://clojure.atlassian.net/browse/CLJS-2472 (reported by aralo)