events: add getMaxListeners method

KhafraDev · web-flow · commit 3904ef8d772a · 2023-03-19T12:58:19.000Z
PR-URL: nodejs#47039 Reviewed-By: Debadree Chatterjee <debadree333@gmail.com> Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
diff --git a/doc/api/events.md b/doc/api/events.md
@@ -1266,6 +1266,58 @@ const { getEventListeners, EventEmitter } = require('node:events');
 }
 ```
 
+## `events.getMaxListeners(emitterOrTarget)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `emitterOrTarget` {EventEmitter|EventTarget}
+* Returns: {number}
+
+Returns the currently set max amount of listeners.
+
+For `EventEmitter`s this behaves exactly the same as calling `.getMaxListeners` on
+the emitter.
+
+For `EventTarget`s this is the only way to get the max event listeners for the
+event target. If the number of event handlers on a single EventTarget exceeds
+the max set, the EventTarget will print a warning.
+
+```mjs
+import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';
+
+{
+  const ee = new EventEmitter();
+  console.log(getMaxListeners(ee)); // 10
+  setMaxListeners(11, ee);
+  console.log(getMaxListeners(ee)); // 11
+}
+{
+  const et = new EventTarget();
+  console.log(getMaxListeners(et)); // 10
+  setMaxListeners(11, et);
+  console.log(getMaxListeners(et)); // 11
+}
+```
+
+```cjs
+const { getMaxListeners, setMaxListeners, EventEmitter } = require('node:events');
+
+{
+  const ee = new EventEmitter();
+  console.log(getMaxListeners(ee)); // 10
+  setMaxListeners(11, ee);
+  console.log(getMaxListeners(ee)); // 11
+}
+{
+  const et = new EventTarget();
+  console.log(getMaxListeners(et)); // 10
+  setMaxListeners(11, et);
+  console.log(getMaxListeners(et)); // 11
+}
+```
+
 ## `events.once(emitter, name[, options])`
 
 <!-- YAML
diff --git a/lib/events.js b/lib/events.js
@@ -220,6 +220,7 @@ module.exports = EventEmitter;
 module.exports.once = once;
 module.exports.on = on;
 module.exports.getEventListeners = getEventListeners;
+module.exports.getMaxListeners = getMaxListeners;
 // Backwards-compat with node 0.10.x
 EventEmitter.EventEmitter = EventEmitter;
 
@@ -930,6 +931,23 @@ function getEventListeners(emitterOrTarget, type) {
                                  emitterOrTarget);
 }
 
+/**
+ * Returns the max listeners set.
+ * @param {EventEmitter | EventTarget} emitterOrTarget
+ * @returns {number}
+ */
+function getMaxListeners(emitterOrTarget) {
+  if (typeof emitterOrTarget?.getMaxListeners === 'function') {
+    return _getMaxListeners(emitterOrTarget);
+  } else if (emitterOrTarget?.[kMaxEventTargetListeners]) {
+    return emitterOrTarget[kMaxEventTargetListeners];
+  }
+
+  throw new ERR_INVALID_ARG_TYPE('emitter',
+                                 ['EventEmitter', 'EventTarget'],
+                                 emitterOrTarget);
+}
+
 /**
  * Creates a `Promise` that is fulfilled when the emitter
  * emits the given event.
diff --git a/test/parallel/test-events-getmaxlisteners.js b/test/parallel/test-events-getmaxlisteners.js
@@ -0,0 +1,19 @@
+'use strict';
+
+require('../common');
+const assert = require('node:assert');
+const { getMaxListeners, EventEmitter, defaultMaxListeners, setMaxListeners } = require('node:events');
+
+{
+  const ee = new EventEmitter();
+  assert.strictEqual(getMaxListeners(ee), defaultMaxListeners);
+  setMaxListeners(101, ee);
+  assert.strictEqual(getMaxListeners(ee), 101);
+}
+
+{
+  const et = new EventTarget();
+  assert.strictEqual(getMaxListeners(et), defaultMaxListeners);
+  setMaxListeners(101, et);
+  assert.strictEqual(getMaxListeners(et), 101);
+}
