From b48473228c3c96933875bc1567c4ecb8b0b10c16 Mon Sep 17 00:00:00 2001
From: Luigi Pinca <luigipinca@gmail.com>
Date: Mon, 26 Oct 2020 16:36:05 +0100
Subject: [PATCH] events: assume an EventEmitter if emitter.on is a function

Assume that the `emitter` argument of `EventEmitter.once()` is an
`EventEmitter` if `emitter.on` is a function.

Refs: https://github.com/nodejs/node/commit/4b3654e923e7c3c2
Refs: https://github.com/websockets/ws/issues/1795

PR-URL: https://github.com/nodejs/node/pull/35818
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
---
 lib/events.js                     |  5 ++++-
 test/parallel/test-events-once.js | 22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/lib/events.js b/lib/events.js
index 51259e6821bf23..300f9f1e6b687f 100644
--- a/lib/events.js
+++ b/lib/events.js
@@ -623,7 +623,10 @@ function unwrapListeners(arr) {
 
 function once(emitter, name) {
   return new Promise((resolve, reject) => {
-    if (typeof emitter.addEventListener === 'function') {
+    if (
+      typeof emitter.addEventListener === 'function' &&
+      typeof emitter.on !== 'function'
+    ) {
       // EventTarget does not have `error` event semantics like Node
       // EventEmitters, we do not listen to `error` events here.
       emitter.addEventListener(
diff --git a/test/parallel/test-events-once.js b/test/parallel/test-events-once.js
index fea143f5877cc7..d07492ea63fa8e 100644
--- a/test/parallel/test-events-once.js
+++ b/test/parallel/test-events-once.js
@@ -166,6 +166,27 @@ async function onceWithEventTargetError() {
   strictEqual(Reflect.has(et.events, 'error'), false);
 }
 
+async function assumesEventEmitterIfOnIsAFunction() {
+  const ee = new EventEmitter();
+  ee.addEventListener = () => {};
+
+  const expected = new Error('kaboom');
+  let err;
+  process.nextTick(() => {
+    ee.emit('error', expected);
+  });
+
+  try {
+    await once(ee, 'myevent');
+  } catch (_e) {
+    err = _e;
+  }
+
+  strictEqual(err, expected);
+  strictEqual(ee.listenerCount('error'), 0);
+  strictEqual(ee.listenerCount('myevent'), 0);
+}
+
 Promise.all([
   onceAnEvent(),
   onceAnEventWithTwoArgs(),
@@ -175,4 +196,5 @@ Promise.all([
   onceWithEventTarget(),
   onceWithEventTargetTwoArgs(),
   onceWithEventTargetError(),
+  assumesEventEmitterIfOnIsAFunction(),
 ]).then(common.mustCall());