ClojureScript setTimeout that fires reliably in a background tab.

set_timeout.cljs

; uses a web worker background thread
(let [metronome-worker-js "self.onmessage=function(e){setTimeout(function(){postMessage(e.data);},e.data.interval);};console.log('Metronome worker loaded.');"
      worker-blob (js/Blob. (clj->js [metronome-worker-js]) {:type "application/javascript"})
      worker (js/Worker. (.createObjectURL js/URL worker-blob))
      call-id (atom 0)]

  (defn make-worker-listener [id callback]
    (fn [e]
      (when (= e.data.id id)
        (callback)
        true)))

  (defn set-timeout [callback interval]
    (let [id (swap! call-id inc)
          listener-fn (make-worker-listener id callback)]
      (.addEventListener worker
                         "message"
                         (fn [e]
                           (when (listener-fn e) (.removeEventListener worker "message" listener-fn)))
                         false)
      (.postMessage worker (clj->js {:id id :interval interval}))))

  ; this emulates async's timeout but in background worker thread
  (defn timeout-worker [interval]
    (let [c (chan)]
      (set-timeout (fn [] (close! c)) interval)
      c)))

(set-timeout (fn [] "Hello!") 1000)

; acts a bit like async's "timeout" but on a backgrounded tab
(go-loop []
         (<! (timeout-worker 500))
         (print "timeout!")
         (recur))

13:58:39.645 timeout 298 core.js:135:8
13:58:39.716 *** worker 958 core.js:135:8
13:58:39.820 timeout 309 core.js:135:8
13:58:40.017 timeout 324 core.js:135:8
13:58:40.027 *** worker 959 core.js:135:8
13:58:40.399 *** worker 960 core.js:135:8
13:58:40.679 timeout 299 core.js:135:8
13:58:40.705 *** worker 961 core.js:135:8

Above "timeout" is using setTimeout but "*** worker" is using (set-timeout)