From 831328bdb295ff4eff9485893cdc70eca88bc656 Mon Sep 17 00:00:00 2001
From: James M Snell <jasnell@gmail.com>
Date: Mon, 6 Jul 2020 08:00:51 -0700
Subject: [PATCH] doc: add note about multiple sync events and once

Fixes: https://github.com/nodejs/node/issues/32431

PR-URL: https://github.com/nodejs/node/pull/34220
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
---
 doc/api/events.md | 54 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/doc/api/events.md b/doc/api/events.md
index 7f4bc71d03f820..eecfa96f8c4312 100644
--- a/doc/api/events.md
+++ b/doc/api/events.md
@@ -884,6 +884,60 @@ ee.emit('error', new Error('boom'));
 // Prints: ok boom
 ```
 
+### Awaiting multiple events emitted on `process.nextTick()`
+
+There is an edge case worth noting when using the `events.once()` function
+to await multiple events emitted on in the same batch of `process.nextTick()`
+operations, or whenever multiple events are emitted synchronously. Specifically,
+because the `process.nextTick()` queue is drained before the `Promise` microtask
+queue, and because `EventEmitter` emits all events synchronously, it is possible
+for `events.once()` to miss an event.
+
+```js
+const { EventEmitter, once } = require('events');
+
+const myEE = new EventEmitter();
+
+async function foo() {
+  await once(myEE, 'bar');
+  console.log('bar');
+
+  // This Promise will never resolve because the 'foo' event will
+  // have already been emitted before the Promise is created.
+  await once(myEE, 'foo');
+  console.log('foo');
+}
+
+process.nextTick(() => {
+  myEE.emit('bar');
+  myEE.emit('foo');
+});
+
+foo().then(() => console.log('done'));
+```
+
+To catch both events, create each of the Promises *before* awaiting either
+of them, then it becomes possible to use `Promise.all()`, `Promise.race()`,
+or `Promise.allSettled()`:
+
+```js
+const { EventEmitter, once } = require('events');
+
+const myEE = new EventEmitter();
+
+async function foo() {
+  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
+  console.log('foo', 'bar');
+}
+
+process.nextTick(() => {
+  myEE.emit('bar');
+  myEE.emit('foo');
+});
+
+foo().then(() => console.log('done'));
+```
+
 ## `events.captureRejections`
 <!-- YAML
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