events: use indexed appends in eventNames

benchmark/events/ee-event-names.js

@@ -0,0 +1,55 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('node:assert');
+const { EventEmitter } = require('node:events');
+
+const bench = common.createBenchmark(main, {
+  n: [1e6],
+  size: [0, 4, 32],
+  shape: ['strings', 'mixed', 'removed'],
+  mode: ['read', 'churn'],
+});
+
+function main({ n, size, shape, mode }) {
+  const emitter = new EventEmitter();
+  const listener = () => {};
+  const keys = [];
+  for (let index = 0; index < size; index++) {
+    const key = shape === 'mixed' && index % 2 === 0 ?
+      Symbol(`event${index}`) : `event${index}`;
+    emitter.on(key, listener);
+    if (shape === 'removed' && index % 2 === 0) {
+      emitter.removeListener(key, listener);
+    } else {
+      keys.push(key);
+    }
+  }
+  if (mode === 'churn') keys.push('transient');
+  const expected = [
+    ...keys.filter((key) => typeof key === 'string'),
+    ...keys.filter((key) => typeof key === 'symbol'),
+  ];
+  const last = expected[expected.length - 1];
+  let checksum = 0;
+  let result;
+  bench.start();
+  if (mode === 'read') {
+    for (let iteration = 0; iteration < n; iteration++) {
+      result = emitter.eventNames();
+      checksum += result.length;
+      if (result.length !== 0 && result[result.length - 1] === last) checksum++;
+    }
+  } else {
+    for (let iteration = 0; iteration < n; iteration++) {
+      emitter.on('transient', listener);
+      result = emitter.eventNames();
+      checksum += result.length;
+      if (result.length !== 0 && result[result.length - 1] === last) checksum++;
+      emitter.removeListener('transient', listener);
+    }
+  }
+  bench.end(n);
+  assert.deepStrictEqual(result, expected);
+  assert.strictEqual(checksum, n * (expected.length + (expected.length !== 0)));
+}

lib/events.js

@@ -877,7 +877,7 @@ EventEmitter.prototype.eventNames = function eventNames() {
   for (const key of ReflectOwnKeys(events)) {
     // Removed listeners leave the key in place with an `undefined` value.
     if (events[key] !== undefined)
-      ArrayPrototypePush(names, key);
+      names[names.length] = key;
   }
   return names;
 };

test/parallel/test-event-emitter-event-names-results.js

@@ -0,0 +1,26 @@
+'use strict';
+
+require('../common');
+const assert = require('node:assert');
+const { EventEmitter } = require('node:events');
+
+const emitter = new EventEmitter();
+const listener = () => {};
+const symbol = Symbol('event');
+const keys = ['third', symbol, '2', '__proto__', 'first', '1'];
+for (const key of keys) emitter.on(key, listener);
+
+const expected = ['1', '2', 'third', '__proto__', 'first', symbol];
+const result = emitter.eventNames();
+assert.deepStrictEqual(result, expected);
+result[0] = 'changed';
+result.push('extra');
+assert.deepStrictEqual(emitter.eventNames(), expected);
+
+emitter.removeListener('2', listener);
+emitter.removeListener('third', listener);
+assert.deepStrictEqual(emitter.eventNames(), ['1', '__proto__', 'first', symbol]);
+emitter.on('third', listener);
+assert.deepStrictEqual(emitter.eventNames(), ['1', '__proto__', 'first', 'third', symbol]);
+emitter.removeAllListeners();
+assert.deepStrictEqual(emitter.eventNames(), []);