JavaScript event loop does NOT "run to completion" - Stack Overflow

According to MDN, messages in the JavaScript event loop "run to completion".
()

But I have created a case where this does not seem to happen.

abortController1 = new AbortController()
abortController2 = new AbortController()

abortController1.signal.addEventListener("abort", () =>
{
    console.log("abort 1 start")
    abortController2.abort()
    console.log("abort 1 end")
})

abortController2.signal.addEventListener("abort", () =>
{
    console.log("abort 2")
})

abortController1.abort()

According to MDN, messages in the JavaScript event loop "run to completion".
(https://developer.mozilla.org/docs/Web/JavaScript/Event_loop#run-to-completion)

But I have created a case where this does not seem to happen.

abortController1 = new AbortController()
abortController2 = new AbortController()

abortController1.signal.addEventListener("abort", () =>
{
    console.log("abort 1 start")
    abortController2.abort()
    console.log("abort 1 end")
})

abortController2.signal.addEventListener("abort", () =>
{
    console.log("abort 2")
})

abortController1.abort()

Output:

abort 1 start
abort 2
abort 1 end

I was expecting to see this output:

abort 1 start
abort 1 end
abort 2

Can someone please explain what's going on here?

EDIT (possible clues) -

• Call to DrainDirectTasks - https://searchfox.org/mozilla-central/rev/94c62970ba2f9c40efd5a4f83a538595425820d9/xpcom/threads/nsThread.cpp#1094
  • https://github.com/mozilla/gecko-dev/blob/26ad7aaffe02c4f9ab84c0faf5bc2f4dc4583806/xpcom/threads/nsThread.cpp#L1094
• Jobs and Host Operations to Enqueue Jobs - https://tc39.es/ecma262/2020/#sec-jobs
• MDN docs EventTarget.dispatchEvent() - https://developer.mozilla.org/docs/Web/API/EventTarget/dispatchEvent
  • Blame - https://github.com/mdn/content/blame/06bf946f61297df045fe24f5fba993aa0c312207/files/en-us/web/api/eventtarget/dispatchevent/index.html#L41

• javascript
• event-loop
• abortcontroller

Share Improve this question Follow edited Jan 7 at 16:50 olfek asked Jan 4 at 22:52 olfekolfek 3,53044 gold badges3737 silver badges5454 bronze badges 2

• I think when you trigger an event explicitly, as with calling abort(), that listener is called synchronously within the current event loop. – Barmar Commented Jan 4 at 22:59
• @Barmar I too was suspecting that, hoping someone can link something to substantiate our suspicions