Explainer: queueMicrotask

Motivation

There are several existing ways to queue a microtask (introduction to microtasks):

Promise.resolve().then(callback)
Register a mutation observer and trigger it with a mutation

The library asap uses these tricks to provide a cross-browser way of queueing a microtask.

Developers and frameworks are already using these methods to queue microtasks. The platform should support queueing microtasks directly as a primitive operation.

Reasons to expose this API directly

Roughly the same semantics are available via Promise etc but we should make this lower level primitive directly available rather than making people emulate it with via higher level APIs
Emulating this via Promise causes exceptions thrown in the callback to be converted into rejected promises.
Emulating via Promise causes unnecessary objects to be created and detroyed. Other methods of emulating this also cause wasted effort.
The other semantically distinct async callback queue APIs are available directly, microtask not being available directly is an anomaly
Providing a direct API could lead to more widespread understanding of the distinction between
- setTimeout(callback,0) - task
- requestAnimationFrame(callback) - RAF
- queueMicrotask(callback) - microtask

Proposal

This proposal would add an API to directly queue a microtask without the need for any tricks. E.g.

queueMicrotask(callback);

This API would be available to JS in web pages via Window and to JS in workers via WorkerGlobalScope.

Usage

Microtasks are useful when you need async ordering behaviour, but want the work done as soon as possible. Beware that doing large amounts of work in microtasks is just as bad for jank and interactivity as doing that work in-place in your JS. Depending on the desired effect, setTimeout(someFunction, 0) or requestAnimationFrame(someFunction) may be the correct choice.

Example

function microtask() {
  console.log("microtask");
}

function task() {
  console.log("task");
}

setTimeout(task, 0);
queueMicrotask(microtask);

Will result in the following order of operations

Log "microtask"
Return to the event loop and allow rendering/input callbacks to occur if appropriate
Log "task"

Risks

Infinite loops of microtasks

A microtask posted with queueMicrotask may itself post another microtask so of course buggy websites can create infinite loops using this API. Since these are microtasks, the current task will never complete, the page will be unresponsive and the slow script dialog will be triggered. This is different to infinite chains of setTimout calls which silently consume 100% CPU but are easier to let slip into production since the tasks complete and event handling continues as normal.

No new mitigations are proposed to handle this risk, the impact of the bug is immediately visible and the slow script dialog seems adequate.

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