events: add eventNames() method

Per #1817, there are many modules
that currently abuse the private `_events` property on EventEmitter.
One of the ways it is used is to determine if a particular event is
being listened for. This adds a simple `eventNames()` method that
returns an array of the events with currently registered listeners.

PR-URL: #5617
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/events.markdown

@@ -295,6 +295,24 @@ they were registered, passing the supplied arguments to each.
 
 Returns `true` if event had listeners, `false` otherwise.
 
+### emitter.eventNames()
+
+Returns an array listing the events for which the emitter has registered
+listeners. The values in the array will be strings or Symbols.
+
+```js
+const EventEmitter = require('events');
+const myEE = new EventEmitter();
+myEE.on('foo', () => {});
+myEE.on('bar', () => {});
+
+const sym = Symbol('symbol');
+myEE.on(sym, () => {});
+
+console.log(myErr.eventNames());
+  // Prints ['foo', 'bar', Symbol('symbol')]
+```
+
 ### emitter.getMaxListeners()
 
 Returns the current max listener value for the `EventEmitter` which is either

lib/events.js

@@ -436,6 +436,15 @@ function listenerCount(type) {
   return 0;
 }
 
+EventEmitter.prototype.eventNames = function eventNames() {
+  if (this._eventsCount > 0) {
+    const events = this._events;
+    return Object.keys(events).concat(
+      Object.getOwnPropertySymbols(events));
+  }
+  return [];
+};
+
 // About 1.5x faster than the two-arg version of Array#splice().
 function spliceOne(list, index) {
   for (var i = index, k = i + 1, n = list.length; k < n; i += 1, k += 1)

test/parallel/test-events-list.js

@@ -0,0 +1,19 @@
+'use strict';
+
+require('../common');
+const EventEmitter = require('events');
+const assert = require('assert');
+
+const EE = new EventEmitter();
+const m = () => {};
+EE.on('foo', () => {});
+assert.deepStrictEqual(['foo'], EE.eventNames());
+EE.on('bar', m);
+assert.deepStrictEqual(['foo', 'bar'], EE.eventNames());
+EE.removeListener('bar', m);
+assert.deepStrictEqual(['foo'], EE.eventNames());
+const s = Symbol('s');
+EE.on(s, m);
+assert.deepStrictEqual(['foo', s], EE.eventNames());
+EE.removeListener(s, m);
+assert.deepStrictEqual(['foo'], EE.eventNames());