mio is doing pretty well as a most-popular event system and reactor. I wouldn&#x27;t be surprised if it&#x27;s blessed in the future, but at the moment the community seems to be there.<p>I&#x27;m also sort of hopeful that higher-kinded types can do something about writing code that can run on any such system. If you can have some functions on an abstract Promise&lt;T&gt; type, like fn then(Promise&lt;T&gt;, fn(T) -&gt; Promise&lt;U&gt;) -&gt; Promise&lt;U&gt;, it doesn&#x27;t super matter which backend you&#x27;re using. (As I understand it, JS is already here, because they can duck-type their promise spec.)