stream: fix eventNames() to not return not defined events

lib/events.js

@@ -49,6 +49,7 @@ const {
   SymbolFor,
   SymbolAsyncIterator,
   SymbolDispose,
+  ArrayIsArray,
 } = primordials;
 const kRejection = SymbolFor('nodejs.rejection');
 

lib/internal/streams/legacy.js

@@ -3,6 +3,7 @@
 const {
   ArrayIsArray,
   ObjectSetPrototypeOf,
+  ReflectOwnKeys,
 } = primordials;
 
 const EE = require('events');
@@ -93,6 +94,16 @@ Stream.prototype.pipe = function(dest, options) {
   return dest;
 };
 
+Stream.prototype.eventNames = function eventNames() {
+  const names = [];
+  for (const key of ReflectOwnKeys(this._events)) {
+    if (typeof this._events[key] === 'function' || (ArrayIsArray(this._events[key]) && this._events[key].length > 0)) {
+      names.push(key);
+    }
+  }
+  return names;
+};
+
 function prependListener(emitter, event, fn) {
   // Sadly this is not cacheable as some libraries bundle their own
   // event emitter implementation with them.

test/parallel/test-stream-event-names.js

@@ -0,0 +1,42 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const { Readable, Writable, Duplex } = require('stream');
+
+{
+  const stream = new Readable();
+  assert.strictEqual(stream.eventNames().length, 0);
+}
+
+{
+  const stream = new Readable();
+  stream.on('foo', () => {});
+  stream.on('data', () => {});
+  stream.on('error', () => {});
+  assert.deepStrictEqual(stream.eventNames(), ['error', 'data', 'foo']);
+}
+
+{
+  const stream = new Writable();
+  assert.strictEqual(stream.eventNames().length, 0);
+}
+
+{
+  const stream = new Writable();
+  stream.on('foo', () => {});
+  stream.on('drain', () => {});
+  stream.on('prefinish', () => {});
+  assert.deepStrictEqual(stream.eventNames(), ['prefinish', 'drain', 'foo']);
+}
+{
+  const stream = new Duplex();
+  assert.strictEqual(stream.eventNames().length, 0);
+}
+
+{
+  const stream = new Duplex();
+  stream.on('foo', () => {});
+  stream.on('finish', () => {});
+  assert.deepStrictEqual(stream.eventNames(), ['finish', 'foo']);
+}