events: avoid retaining removed event names

lib/events.js

@@ -86,6 +86,7 @@ const { addAbortListener } = require('internal/events/abort_listener');
 
 const kCapture = Symbol('kCapture');
 const kErrorMonitor = Symbol('events.errorMonitor');
+const kShapeMode = Symbol('shapeMode');
 const kEmitting = Symbol('events.emitting');
 const kMaxEventTargetListeners = Symbol('events.maxEventTargetListeners');
 const kMaxEventTargetListenersWarned =
@@ -334,6 +335,9 @@ EventEmitter.init = function(opts) {
       this._events === ObjectGetPrototypeOf(this)._events) {
     this._events = { __proto__: null };
     this._eventsCount = 0;
+    this[kShapeMode] = false;
+  } else {
+    this[kShapeMode] = true;
   }
 
   this._maxListeners ||= undefined;
@@ -687,12 +691,13 @@ EventEmitter.prototype.removeListener =
       if (list === listener || list.listener === listener) {
         this._eventsCount -= 1;
 
-        // Leave the key in place with an `undefined` value: repeatedly
-        // adding and removing a listener for the same event this way keeps
-        // the `events` object in the same shape and avoids both a `delete`
-        // (which would put the object into dictionary mode) and allocating
-        // a fresh object when the last listener is removed.
-        events[type] = undefined;
+        if (this[kShapeMode]) {
+          events[type] = undefined;
+        } else if (this._eventsCount === 0) {
+          this._events = { __proto__: null };
+        } else {
+          delete events[type];
+        }
 
         if (events.removeListener !== undefined)
           this.emit('removeListener', type, list.listener || listener);
@@ -752,6 +757,7 @@ EventEmitter.prototype.removeAllListeners =
           else
             delete events[type];
         }
+        this[kShapeMode] = false;
         return this;
       }
 
@@ -764,6 +770,7 @@ EventEmitter.prototype.removeAllListeners =
         this.removeAllListeners('removeListener');
         this._events = { __proto__: null };
         this._eventsCount = 0;
+        this[kShapeMode] = false;
         return this;
       }
 

test/parallel/test-event-emitter-remove-listeners.js

@@ -144,6 +144,19 @@ function listener2() {}
   assert.deepStrictEqual(ee, ee.removeListener('foo', () => {}));
 }
 
+{
+  const ee = new EventEmitter();
+  ee.on('persistent', listener1);
+
+  for (let i = 0; i < 100; i++) {
+    const eventName = `event-${i}`;
+    ee.on(eventName, listener2);
+    ee.removeListener(eventName, listener2);
+  }
+
+  assert.deepStrictEqual(Reflect.ownKeys(ee._events), ['persistent']);
+}
+
 {
   const ee = new EventEmitter();
   const listener = () => {};