Clarifying the semantics of Async<'a> on the JS runtime (queues, Sleep, rAF, errors/cancellation)

[eagle-k]

As a day-to-day Fable user, I’d like to confirm how Async<'a> is intended to behave on the JavaScript runtime (in Fable), and propose adding a short doc page if the current behavior is considered the spec.

In .NET, Async<'a> describes an asynchronous computation independent of how it’s executed (not tied to threads). On JS we have a single-threaded event loop with a microtask queue and a macrotask queue, which makes it easy to get confused about what runs where.

What I’m trying to confirm

1. After let! / do!, which queue runs the continuation on JS?
2. How Async.Sleep ms is scheduled (macrotask?).
3. Recommended way to align an Async workflow with requestAnimationFrame.
4. A short note on error propagation and cancellation.
5. JS-specific limitations worth calling out (no RunSynchronously, no true multithreading, etc.).

My current mental model (please correct if wrong)

• let! / do! behave like a Promise .then chain, so continuations run as microtasks.
• Async.Sleep ms uses a timer (e.g., setTimeout), so it resumes as a macrotask.
• For UI, requestAnimationFrame matters; there’s no built-in Async primitive for it.

Why this would help

• Predictability for UI- and perf-sensitive code
• Fewer repeated Q&A about queues and timing
• A shared semantic baseline for F# Async on JS (useful for both users and contributors)

[MangelMaxime]

Hello,

Sorry I don't have enough knowledge regarding microtasks and macrotask to answer your question.

If it helps, the implementation of Async can be found here and for async CE here

You can use this file and Fable REPL to see how the code is translated.

Note: I know that in general F# people prefer to use Async but it feels familiar to the standard F# code, but personally I prefer to use promise via Fable.Promise because they are better integrated/native with JavaScript. Which also means we can rely on JavaScript documentation/code to answer the kind of question your asked above.