events: optimize once() and removeListener()

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)

lib/events.js

@@ -86,7 +86,6 @@ 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 =
@@ -335,9 +334,6 @@ 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;
@@ -446,6 +442,39 @@ function enhanceStackTrace(err, own) {
  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
 }
 
+function getUnhandledErrorException(ee, args) {
+ let er;
+ if (args.length > 0)
+ er = args[0];
+ if (er instanceof Error) {
+ try {
+ const capture = {};
+ ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
+ ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
+ __proto__: null,
+ value: FunctionPrototypeBind(enhanceStackTrace, ee, er, capture),
+ configurable: true,
+ });
+ } catch {
+ // Continue regardless of error.
+ }
+
+ return er;
+ }
+
+ let stringifiedEr;
+ try {
+ stringifiedEr = inspect(er);
+ } catch {
+ stringifiedEr = er;
+ }
+
+ // At least give some kind of context to the user
+ const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
+ err.context = er;
+ return err;
+}
+
 /**
  * Synchronously calls each of the listeners registered
  * for the event.
@@ -466,38 +495,10 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- let er;
- if (args.length > 0)
- er = args[0];
- if (er instanceof Error) {
- try {
- const capture = {};
- ErrorCaptureStackTrace(capture, EventEmitter.prototype.emit);
- ObjectDefineProperty(er, kEnhanceStackBeforeInspector, {
- __proto__: null,
- value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
- configurable: true,
- });
- } catch {
- // Continue regardless of error.
- }
-
- // Note: The comments on the `throw` lines are intentional, they show
- // up in Node's output if this results in an unhandled exception.
- throw er; // Unhandled 'error' event
- }
-
- let stringifiedEr;
- try {
- stringifiedEr = inspect(er);
- } catch {
- stringifiedEr = er;
- }
-
- // At least give some kind of context to the user
- const err = new ERR_UNHANDLED_ERROR(stringifiedEr);
- err.context = er;
- throw err; // Unhandled 'error' event
+ const er = getUnhandledErrorException(this, args);
+ // Note: The comments on the `throw` lines are intentional, they show
+ // up in Node's output if this results in an unhandled exception.
+ throw er; // Unhandled 'error' event
  }
 
  const handler = events[type];
@@ -584,20 +585,23 @@ function _addListener(target, type, listener, prepend) {
 
  // Check for listener leak
  m = _getMaxListeners(target);
- if (m > 0 && existing.length > m && !existing.warned) {
- existing.warned = true;
- // No error code for this since it is a Warning
- const w = genericNodeError(
- `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
- `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
- { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
- process.emitWarning(w);
- }
+ if (m > 0 && existing.length > m && !existing.warned)
+ warnMaxListenersExceeded(target, type, existing, m);
  }
 
  return target;
 }
 
+function warnMaxListenersExceeded(target, type, existing, m) {
+ existing.warned = true;
+ // No error code for this since it is a Warning
+ const w = genericNodeError(
+ `Possible EventEmitter memory leak detected. ${existing.length} ${String(type)} listeners ` +
+ `added to ${inspect(target, { depth: -1 })}. MaxListeners is ${m}. Use emitter.setMaxListeners() to increase limit`,
+ { name: 'MaxListenersExceededWarning', emitter: target, type: type, count: existing.length });
+ process.emitWarning(w);
+}
+
 /**
  * Adds a listener to the event emitter.
  * @param {string | symbol} type
@@ -622,22 +626,16 @@ EventEmitter.prototype.prependListener =
  return _addListener(this, type, listener, true);
  };
 
-function onceWrapper() {
- if (!this.fired) {
- this.target.removeListener(this.type, this.wrapFn);
- this.fired = true;
- if (arguments.length === 0)
- return this.listener.call(this.target);
- return ReflectApply(this.listener, this.target, arguments);
- }
-}
-
 function _onceWrap(target, type, listener) {
- const state = { fired: false, wrapFn: undefined, target, type, listener };
- const wrapped = onceWrapper.bind(state);
- wrapped.listener = listener;
- state.wrapFn = wrapped;
- return wrapped;
+ let fired = false;
+ function wrapper(...args) {
+ if (fired) return;
+ fired = true;
+ target.removeListener(type, wrapper);
+ return ReflectApply(listener, target, args);
+ }
+ wrapper.listener = listener;
+ return wrapper;
 }
 
 /**
@@ -689,13 +687,12 @@ EventEmitter.prototype.removeListener =
  if (list === listener || list.listener === listener) {
  this._eventsCount -= 1;
 
- if (this[kShapeMode]) {
- events[type] = undefined;
- } else if (this._eventsCount === 0) {
- this._events = { __proto__: null };
- } else {
- delete events[type];
- }
+ // 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 (events.removeListener !== undefined)
  this.emit('removeListener', type, list.listener || listener);
@@ -755,7 +752,6 @@ EventEmitter.prototype.removeAllListeners =
  else
  delete events[type];
  }
- this[kShapeMode] = false;
  return this;
  }
 
@@ -768,7 +764,6 @@ EventEmitter.prototype.removeAllListeners =
  this.removeAllListeners('removeListener');
  this._events = { __proto__: null };
  this._eventsCount = 0;
- this[kShapeMode] = false;
  return this;
  }
 
@@ -868,7 +863,16 @@ EventEmitter.prototype.listenerCount = function listenerCount(type, listener) {
  * @returns {(string | symbol)[]}
  */
 EventEmitter.prototype.eventNames = function eventNames() {
- return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
+ if (this._eventsCount === 0)
+ return [];
+ const events = this._events;
+ const names = [];
+ for (const key of ReflectOwnKeys(events)) {
+ // Removed listeners leave the key in place with an `undefined` value.
+ if (events[key] !== undefined)
+ ArrayPrototypePush(names, key);
+ }
+ return names;
 };
 
 function arrayClone(arr) {

test/fixtures/errors/events_unhandled_error_common_trace.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error: foo:bar
  at bar (<project-root>/test/fixtures/errors/events_unhandled_error_common_trace.js:9:12)

test/fixtures/errors/events_unhandled_error_nexttick.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_nexttick.js:6:12)

test/fixtures/errors/events_unhandled_error_sameline.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_sameline.js:6:34)

test/fixtures/errors/events_unhandled_error_subclass.snapshot

@@ -1,6 +1,6 @@
 node:events:<line>
-throw er; // Unhandled 'error' event
-^
+ throw er; // Unhandled 'error' event
+ ^
 
 Error
  at Object.<anonymous> (<project-root>/test/fixtures/errors/events_unhandled_error_subclass.js:7:25)