queueMicrotask() is called before process.nextTick() in ESM

[deokjinkim]

Version

v20.0.0-pre

Platform

Linux deokjinkim-MS-7885 5.15.0-67-generic #74-Ubuntu SMP Wed Feb 22 14:14:39 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

process

What steps will reproduce the bug?

When I run example of ESM in document, actual result is different from expected result.
https://nodejs.org/dist/latest-v19.x/docs/api/process.html#when-to-use-queuemicrotask-vs-processnexttick

import { nextTick } from 'node:process';

Promise.resolve().then(() => console.log(2));
queueMicrotask(() => console.log(3));
nextTick(() => console.log(1));
// Output:
// 1
// 2
// 3

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

1
2
3

What do you see instead?

2
3
1

Additional information

In example of CJS, actual result is same to expected result.

const { nextTick } = require('node:process');

Promise.resolve().then(() => console.log(2));
queueMicrotask(() => console.log(3));
nextTick(() => console.log(1));
// Output:
// 1
// 2
// 3

[jasnell]

Yes, this is a known and correct behavior due to the fact that an ESM entry point is run after the event loop starts and is processed as part of the microtask queue. It's based on the internal ordering of microtasks vs. next ticks.

With CommonJS, scripts are run as a function. That function queues one next tick and one microtask. When the stack unwinds back to c++, we first drain the next tick queue followed by the microtask queue, which means the next tick callback is invoked first, leading to the order (1, 2, 3).

With ESM, there is a promise, and the code is loaded within the scope of a microtask. Since the code schedules another microtask while we are in the process of draining the queue, we end up calling the queued microtasks first (so we get the 2 and 3), and only after that stack unwinds back to c++ do we drain the pending next tick queue (getting the 1).

I'm glossing over a number of details here but what you're seeing here is absolutely expected behavior that is definitely not a bug.

[deokjinkim]

@jasnell Thank you for kind explanation. Then, do we need to update example of ESM because (1, 2, 3) is expected in document for now? According to your explanation, (2, 3, 1) is expected for example of ESM in below document.
https://nodejs.org/dist/latest-v19.x/docs/api/process.html#when-to-use-queuemicrotask-vs-processnexttick

[climba03003]

Duplicate of #45048
PR that stale #45093 for fixing the example

[jasnell]

Yeah that example needs to be updated