An implementation of ExecutionContext in terms of JavaScript's setImmediate. Unfortunately for everyone involved, setImmediate is only available on Edge and Node.js, meaning that this functionality must be polyfilled on all other environments. The details of this polyfill can be found in the readme of the excellent YuzuJS/setImmediate project, though the implementation here is in terms of Scala.js primitives rather than raw JavaScript.

Unless you have some very, very specific and unusual requirements, this is the optimal ExecutionContext implementation for use in any Scala.js project. If you're using ExecutionContext and not using this project, you likely have some serious bugs and/or performance issues waiting to be discovered.


libraryDependencies += "org.scala-js" %%% "scala-js-macrotask-executor" % "1.1.1"

Published for Scala 2.11, 2.12, 2.13, 3. Functionality is fully supported on all platforms supported by Scala.js (including web workers). In the event that a given platform does not have the necessary functionality to implement setImmediate-style yielding (usually postMessage is what is required), the implementation will transparently fall back to using setTimeout, which will drastically inhibit performance but remain otherwise functional.

import org.scalajs.macrotaskexecutor.MacrotaskExecutor.Implicits._

You can also simply import MacrotaskExecutor if using the ExecutionContext directly.

Once imported, this executor functions exactly the same as, except it does not suffer from the various limitations of a Promise- or setTimeout-based implementation. In other words, you can use Future (and other ExecutionContext-based tooling) effectively exactly as you would on the JVM, and it will behave effectively identically modulo the single-threaded nature of the runtime.


The original motivation for this functionality comes from the following case (written here in terms of Future, but originally discovered in terms of IO in the Cats Effect project):

var cancel = false

def loop(): Future[Unit] =
  Future(cancel) flatMap { canceled =>
    if (canceled)

js.timers.setTimeout(100.millis) {
  cancel = true


The loop() future will run forever when using the default Scala.js executor, which is written in terms of JavaScript's Promise. The reason this will run forever stems from the fact that JavaScript includes two separate work queues: the microtask and the macrotask queue. The microtask queue is used exclusively by Promise, while the macrotask queue is used by everything else, including UI rendering, setTimeout, and I/O such as Fetch or Node.js things. The semantics are such that, whenever the microtask queue has work, it takes full precedence over the macrotask queue until the microtask queue is completely exhausted.

This explains why the above snippet will run forever on a Promise-based executor: the microtask queue is never empty because we're constantly adding new tasks! Thus, setTimeout is never able to run because the macrotask queue never receives control.


This is fixable by using a setTimeout-based executor, such as the QueueExecutionContext.timeouts() implementation in Scala.js. Available in all browsers since the dawn of time, setTimeout takes two arguments: a time delay and a callback to invoke. The callback is invoked by the event loop once the time delay expires, and this is implemented by pushing the callback onto the back of the event queue at the appropriate time. Calling setTimeout with a delay of 0 would seem to achieve exactly the semantics we want: yield back to the event loop and allow it to resume our callback when it's our turn once again.

Unfortunately, setTimeout is slow. Very, very, very slow. The timing mechanism imposes quite a bit of overhead, even when the delay is 0, and there are other complexities which ultimately impose a performance penalty too severe to accept. Any significant application of an ExecutionContext backed by setTimeout, would be almost unusable.

To make matters worse, setTimeout is clamped in all JavaScript environments. In particular, it is clamped to a minimum of 4ms and, in practice, usually somewhere between 4ms and 10ms. This clamping kicks in whenever more than 5 consecutive timeouts have been scheduled:

setTimeout(() => {
  setTimeout(() => {
    setTimeout(() => {
      setTimeout(() => {
        setTimeout(() => {
          // this one (and all after it) are clamped!
        }, 0);
      }, 0);
    }, 0);
  }, 0);
}, 0);

Each timeout sets a new timeout, and so on and so on. This is exactly the sort of situation that we get into when chaining Futures, where each map/flatMap/transform/etc. schedules another Future which, in turn will schedule another... etc. etc. This is exactly where we see clamping. In particular, the innermost setTimeout in this example will be clamped to 4 milliseconds (meaning there is no difference between setTimeout(.., 0) and setTimeout(.., 4)), which would slow down execution even more.

You can read more details in the MDN documentation.


Fortunately, we aren't the only ones to have this problem. What we want is something which uses the macrotask queue (so we play nicely with setTimeout, I/O, and other macrotasks), but which doesn't have as much overhead as setTimeout. The answer is setImmediate.

The setImmediate function was first introduced in Node.js, and its purpose is to solve exactly this problem: a faster setTimeout(..., 0). In particular, setImmediate(...) is semantically equivalent to setTimeout(0, ...), except without the associated clamping: it doesn't include a delay mechanism of any sort, it simply takes a callback and immediately submits it to the event loop, which in turn will run the callback as soon as its turn comes up.

Unfortunately, setImmediate isn't available on every platform. For reasons of... their own, Mozilla, Google, and Apple have all strenuously objected to the inclusion of setImmediate in the W3C standard set, despite the proposal (which originated at Microsoft) and obvious usefulness. This in turn has resulted in inconsistency across the JavaScript space.

That's the bad news. The good news is that all modern browsers include some sort of functionality which can be exploited to emulate setImmediate with similar performance characteristics. In particular, most environments take advantage of postMessage in some way. If you're interested in the nitty-gritty details of how this works, you are referred to this excellent readme.

scala-js-macrotask-executor implements most of the setImmediate polyfill in terms of Scala.js, wrapped up in an ExecutionContext interface. The only elements of the polyfill which are not implemented are as follows:

  • process.nextTick is used by the JavaScript polyfill when running on Node.js versions below 0.9. However, Scala.js itself does not support Node.js 0.9 or below, so there's really no point in supporting this case.
  • Similarly, older versions of IE (6 through 8, specifically) allow a particular exploitation of the onreadystatechange event fired when a <script> element is inserted into the DOM. However, Scala.js does not support these environments either, and so there is no benefit to implementing this case.

On environments where the polyfill is unsupported, setTimeout is still used as a final fallback.

Performance Notes

Optimal performance is currently available in the following environments:

setImmediate in practice seems to be somewhat slower than Promise.then(), particularly on Chrome. However, since Promise also has seriously detrimental effects (such as blocking UI rendering), it doesn't seem to be a particularly fair comparison. Promise is also slower than setImmediate on Firefox for very unclear reasons likely having to do with fairness issues in the Gecko engine itself.

setImmediate is dramatically faster than setTimeout, mostly due to clamping but also because setTimeout has other sources of overhead. In particular, executing 10,000 sequential tasks takes about 30 seconds with setTimeout and about 400 milliseconds using setImmediate.

See scala-js#4129 for additional background discussion.