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Make No Promises

23 August 2013

Promises are all the rage in the JavaScript world even though they don't actually eliminate callback hell and their design emphasizes coarse grained asychronous operations leaving much to be desired for the vast universe of possible interactive applications (cough, user interfaces).

Here is a simple benchmark using one of the fastest promise implementations, when.js.

function goWhen() {
  var first = when.defer(), last = first.promise;

  for(var i = 0; i < 100000; i++) {
    last = last.then(function(val) {
      return val + 1;

  var s = new Date();
  last.then(function(val) {
    var el = document.getElementById("when-time")
    el.innerHTML = val + " elapsed ms: " + (new Date()-s);

The same conceptual thing in core.async:

(defn ^:export go-cljs []
  (let [first (chan)
        last (loop [i 0 last first]
               (if (< i 100000)
                 (let [next (chan)]
                   (take! last (fn [v] (put! next (inc v))))
                   (recur (inc i) next))
    (go (let [s  (js/Date.)
              el (.getElementById js/document "cljs-time")]
          (>! first 0)
          (set! (.-innerHTML el)
            (str (<! last) " elapsed ms: " (- (js/Date.) s)))))))

As you can see core.async is competitive. However it has the advantage in that we can create channels and do many asynchronous operations over them instead of wastefully instantiating promises again and again. We can also combine channels with go blocks freeing us from callback hell.

The above performance is the result of several improvements we've landed in core.async this week, and what follows are some other highlights.

On a 1.7ghz MacBook Air running Chrome Canary we can push one million events down a channel in around 1 second.

The following code pushes an event down a 100,000 channel long daisy chain in less than 200ms in Chrome Canary. For comparison this takes about 60-80ms in Go on my machine and of course I can't run Go in my web browser.

In Node.js just calling setImmediate 100,000 times takes 360ms. The following code takes ~420ms under Node.js. That's an incredibly small amount of overhead.

(defn f [left right]
  (go (>! left (inc (<! right)))))

(let [leftmost (chan)
      rightmost (loop [n 100000 left leftmost]
                  (if-not (pos? n)
                    (let [right (chan)]
                      (f left right)
                      (recur (dec n) right))))]
    (let [s (js/Date.)]
      (>! rightmost 1)
      (.log js/console (<! leftmost) " elapsed ms: "
        (- (js/Date.) s)))))

I'm sure core.async performance will continue to improve but these examples demonstrate that core.async can likely handle even the most demanding complex event driven applications while providing one of the highest level of abstractions offered by any language targeting JavaScript today.