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: nodejs@4b3654e923e7c3c2
Refs: websockets/ws#1795

Author: lpinca

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(

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());