Inconsistent behavior of nextTick and queueMicrotask

Consider this snippet:

const { EventEmitter } = require('node:events')

function run (name) {
  return new Promise((resolve) => {
    const e = new EventEmitter()
    process.nextTick(() => {
      e.emit('error', new Error())
    });
    resolve(e)
  }).then(((e) => {
    e.on('error', err => {
      console.error(`### ${name} ###`, err)
    })
  }))
}

queueMicrotask(run.bind(null, 'queueMicrotask'))
setImmediate(run.bind(null, 'setImmediate'))

This results in

### queueMicrotask ### Error
    at /Users/matteo/c.cjs:7:23
    at process.processTicksAndRejections (node:internal/process/task_queues:77:11)
node:events:492
      throw er; // Unhandled 'error' event
      ^

Error
    at /Users/matteo/c.cjs:7:23
    at process.processTicksAndRejections (node:internal/process/task_queues:77:11)
Emitted 'error' event at:
    at /Users/matteo/c.cjs:7:9
    at process.processTicksAndRejections (node:internal/process/task_queues:77:11)

Node.js v20.10.0

Why is this an issue? In most part of Node.js core,
we use process.nextTick(() => ee.emit('error')) to let users install event handlers.
However, if an EventEmitter is returned by an async function, there is a significant possibility that the error could not be caught.

(This is essentially a problem for Node.js streams).

@jasnell

@jasnell what do you think?

Just to expand a bit. There is an assumption that process.nextTick tasks always run before micro-tasks. However, as demonstrated above. This is a false assumption. The reality is more inconsistent and depends whether or not the caller is inside our outside a micro tick.

I'm adding this to the TSC agenda for visibility and help with triaging the severity of the issue.

Would probably be a good idea to update https://nodejs.org/en/guides/event-loop-timers-and-nexttick to also include promises and microticks.

It's not entirely clear to me what is the expected output in the OP, did you mean to say that you expect a ### setImmediate ### Error log in addition to the queueMicrotask one? Or did you mean to expect to see just the setImmediate one?

It's not entirely clear to me what is the expected output in the OP, did you mean to say that you expect a ### setImmediate ### Error log in addition to the queueMicrotask one? Or did you mean to expect to see just the setImmediate one?

The expectation is that:

queueMicrotask(run.bind(null, 'queueMicrotask'))

and

setImmediate(run.bind(null, 'setImmediate'))

Should have the same output, which they don't.

You mean neither of them should log anything or they should both log or either is fine as long as they are the same? I am guessing from the OP (in the context of error handling) that you want both to log?

I think you could argue that either behavior is correct (or at least not incorrect).

The problem here is that they are not consistent.

@ronag

Hello @ronag, I would like to continue working on this issue, however, as mentioned on your PR, I guess a simple search and replace won't work?
I am actually new to contributing to NodeJS, so I am a little confused as to what we can implement here. If you could give me some pointers, it would be of great help.

@NiharPhansalkar

@NiharPhansalkar I don't think this is a good starter issue. Not even us in the technical steering are sure what to do about this issue.

@benjamingr

@benjamingr here is the patch that fixes the behavior... but it breaks a lot of tests:

diff --git a/lib/internal/process/task_queues.js b/lib/internal/process/task_queues.js
index bcb5eef841..04bdb07122 100644
--- a/lib/internal/process/task_queues.js
+++ b/lib/internal/process/task_queues.js
@@ -67,6 +67,7 @@ function runNextTicks() {
 function processTicksAndRejections() {
   let tock;
   do {
+    runMicrotasks();
     while ((tock = queue.shift()) !== null) {
       const asyncId = tock[async_id_symbol];
       emitBefore(asyncId, tock[trigger_async_id_symbol], tock);
@@ -92,7 +93,6 @@ function processTicksAndRejections() {

       emitAfter(asyncId);
     }
-    runMicrotasks();
   } while (!queue.isEmpty() || processPromiseRejections());
   setHasTickScheduled(false);
   setHasRejectionToWarn(false);

doc: mark process.nextTick legacy

This issue has been marked as stale due to 210 days of inactivity.
It will be automatically closed in 30 days if no further activity occurs. If this is still relevant, please leave a comment or update it to keep it open.

@mcollina

@mcollina completed?

I just pressed a single button on mobile.

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Inconsistent behavior of nextTick and queueMicrotask #51156

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Inconsistent behavior of nextTick and queueMicrotask #51156

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mcollina commented on Dec 14, 2023

ronag commented on Dec 14, 2023

ronag commented on Dec 18, 2023

ronag commented on Dec 18, 2023

joyeecheung commented on Dec 18, 2023

ronag commented on Dec 18, 2023

joyeecheung commented on Dec 18, 2023

ronag commented on Dec 18, 2023

NiharPhansalkar commented on Dec 19, 2023

ronag commented on Dec 19, 2023

mcollina commented on Dec 20, 2023

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github-actions commented on May 24, 2026

ronag commented on May 24, 2026

mcollina commented on May 25, 2026

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