events.js

// Copyright Joyent, Inc. and other Node contributors.
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to permit
// persons to whom the Software is furnished to do so, subject to the
// following conditions:
//
// The above copyright notice and this permission notice shall be included
// in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
// USE OR OTHER DEALINGS IN THE SOFTWARE.

'use strict';

const {
  ArrayPrototypeIndexOf,
  ArrayPrototypeJoin,
  ArrayPrototypeShift,
  ArrayPrototypeSlice,
  ArrayPrototypeSplice,
  Boolean,
  Error,
  ErrorCaptureStackTrace,
  FunctionPrototypeBind,
  FunctionPrototypeCall,
  MathMin,
  NumberIsNaN,
  ObjectCreate,
  ObjectDefineProperty,
  ObjectDefineProperties,
  ObjectGetPrototypeOf,
  ObjectSetPrototypeOf,
  Promise,
  PromiseReject,
  PromiseResolve,
  ReflectOwnKeys,
  String,
  StringPrototypeSplit,
  Symbol,
  SymbolFor,
  SymbolAsyncIterator,
} = primordials;
const kRejection = SymbolFor('nodejs.rejection');

let spliceOne;

const {
  AbortError,
  kEnhanceStackBeforeInspector,
  codes: {
    ERR_INVALID_ARG_TYPE,
    ERR_OUT_OF_RANGE,
    ERR_UNHANDLED_ERROR
  },
} = require('internal/errors');

const {
  inspect
} = require('internal/util/inspect');

const {
  validateAbortSignal,
  validateBoolean,
  validateFunction,
} = require('internal/validators');

const kCapture = Symbol('kCapture');
const kErrorMonitor = Symbol('events.errorMonitor');
const kMaxEventTargetListeners = Symbol('events.maxEventTargetListeners');
const kMaxEventTargetListenersWarned =
  Symbol('events.maxEventTargetListenersWarned');

/**
 * Creates a new `EventEmitter` instance.
 * @param {{ captureRejections?: boolean; }} [opts]
 * @returns {EventEmitter}
 */
function EventEmitter(opts) {
  EventEmitter.init.call(this, opts);
}
module.exports = EventEmitter;
module.exports.once = once;
module.exports.on = on;
module.exports.getEventListeners = getEventListeners;
// Backwards-compat with node 0.10.x
EventEmitter.EventEmitter = EventEmitter;

EventEmitter.usingDomains = false;

EventEmitter.captureRejectionSymbol = kRejection;
ObjectDefineProperty(EventEmitter, 'captureRejections', {
  get() {
    return EventEmitter.prototype[kCapture];
  },
  set(value) {
    validateBoolean(value, 'EventEmitter.captureRejections');

    EventEmitter.prototype[kCapture] = value;
  },
  enumerable: true
});

EventEmitter.errorMonitor = kErrorMonitor;

// The default for captureRejections is false
ObjectDefineProperty(EventEmitter.prototype, kCapture, {
  value: false,
  writable: true,
  enumerable: false
});

EventEmitter.prototype._events = undefined;
EventEmitter.prototype._eventsCount = 0;
EventEmitter.prototype._maxListeners = undefined;

// By default EventEmitters will print a warning if more than 10 listeners are
// added to it. This is a useful default which helps finding memory leaks.
let defaultMaxListeners = 10;
let isEventTarget;

function checkListener(listener) {
  validateFunction(listener, 'listener');
}

ObjectDefineProperty(EventEmitter, 'defaultMaxListeners', {
  enumerable: true,
  get: function() {
    return defaultMaxListeners;
  },
  set: function(arg) {
    if (typeof arg !== 'number' || arg < 0 || NumberIsNaN(arg)) {
      throw new ERR_OUT_OF_RANGE('defaultMaxListeners',
                                 'a non-negative number',
                                 arg);
    }
    defaultMaxListeners = arg;
  }
});

ObjectDefineProperties(EventEmitter, {
  kMaxEventTargetListeners: {
    value: kMaxEventTargetListeners,
    enumerable: false,
    configurable: false,
    writable: false,
  },
  kMaxEventTargetListenersWarned: {
    value: kMaxEventTargetListenersWarned,
    enumerable: false,
    configurable: false,
    writable: false,
  }
});

/**
 * Sets the max listeners.
 * @param {number} n
 * @param {EventTarget[] | EventEmitter[]} [eventTargets]
 * @returns {void}
 */
EventEmitter.setMaxListeners =
  function(n = defaultMaxListeners, ...eventTargets) {
    if (typeof n !== 'number' || n < 0 || NumberIsNaN(n))
      throw new ERR_OUT_OF_RANGE('n', 'a non-negative number', n);
    if (eventTargets.length === 0) {
      defaultMaxListeners = n;
    } else {
      if (isEventTarget === undefined)
        isEventTarget = require('internal/event_target').isEventTarget;

      for (let i = 0; i < eventTargets.length; i++) {
        const target = eventTargets[i];
        if (isEventTarget(target)) {
          target[kMaxEventTargetListeners] = n;
          target[kMaxEventTargetListenersWarned] = false;
        } else if (typeof target.setMaxListeners === 'function') {
          target.setMaxListeners(n);
        } else {
          throw new ERR_INVALID_ARG_TYPE(
            'eventTargets',
            ['EventEmitter', 'EventTarget'],
            target);
        }
      }
    }
  };

EventEmitter.init = function(opts) {

  if (this._events === undefined ||
      this._events === ObjectGetPrototypeOf(this)._events) {
    this._events = ObjectCreate(null);
    this._eventsCount = 0;
  }

  this._maxListeners = this._maxListeners || undefined;


  if (opts?.captureRejections) {
    validateBoolean(opts.captureRejections, 'options.captureRejections');
    this[kCapture] = Boolean(opts.captureRejections);
  } else {
    // Assigning the kCapture property directly saves an expensive
    // prototype lookup in a very sensitive hot path.
    this[kCapture] = EventEmitter.prototype[kCapture];
  }
};

function addCatch(that, promise, type, args) {
  if (!that[kCapture]) {
    return;
  }

  // Handle Promises/A+ spec, then could be a getter
  // that throws on second use.
  try {
    const then = promise.then;

    if (typeof then === 'function') {
      then.call(promise, undefined, function(err) {
        // The callback is called with nextTick to avoid a follow-up
        // rejection from this promise.
        process.nextTick(emitUnhandledRejectionOrErr, that, err, type, args);
      });
    }
  } catch (err) {
    that.emit('error', err);
  }
}

function emitUnhandledRejectionOrErr(ee, err, type, args) {
  if (typeof ee[kRejection] === 'function') {
    ee[kRejection](err, type, ...args);
  } else {
    // We have to disable the capture rejections mechanism, otherwise
    // we might end up in an infinite loop.
    const prev = ee[kCapture];

    // If the error handler throws, it is not catcheable and it
    // will end up in 'uncaughtException'. We restore the previous
    // value of kCapture in case the uncaughtException is present
    // and the exception is handled.
    try {
      ee[kCapture] = false;
      ee.emit('error', err);
    } finally {
      ee[kCapture] = prev;
    }
  }
}

/**
 * Increases the max listeners of the event emitter.
 * @param {number} n
 * @returns {EventEmitter}
 */
EventEmitter.prototype.setMaxListeners = function setMaxListeners(n) {
  if (typeof n !== 'number' || n < 0 || NumberIsNaN(n)) {
    throw new ERR_OUT_OF_RANGE('n', 'a non-negative number', n);
  }
  this._maxListeners = n;
  return this;
};

function _getMaxListeners(that) {
  if (that._maxListeners === undefined)
    return EventEmitter.defaultMaxListeners;
  return that._maxListeners;
}

/**
 * Returns the current max listener value for the event emitter.
 * @returns {number}
 */
EventEmitter.prototype.getMaxListeners = function getMaxListeners() {
  return _getMaxListeners(this);
};

// Returns the length and line number of the first sequence of `a` that fully
// appears in `b` with a length of at least 4.
function identicalSequenceRange(a, b) {
  for (let i = 0; i < a.length - 3; i++) {
    // Find the first entry of b that matches the current entry of a.
    const pos = ArrayPrototypeIndexOf(b, a[i]);
    if (pos !== -1) {
      const rest = b.length - pos;
      if (rest > 3) {
        let len = 1;
        const maxLen = MathMin(a.length - i, rest);
        // Count the number of consecutive entries.
        while (maxLen > len && a[i + len] === b[pos + len]) {
          len++;
        }
        if (len > 3) {
          return [len, i];
        }
      }
    }
  }

  return [0, 0];
}

function enhanceStackTrace(err, own) {
  let ctorInfo = '';
  try {
    const { name } = this.constructor;
    if (name !== 'EventEmitter')
      ctorInfo = ` on ${name} instance`;
  } catch {}
  const sep = `\nEmitted 'error' event${ctorInfo} at:\n`;

  const errStack = ArrayPrototypeSlice(
    StringPrototypeSplit(err.stack, '\n'), 1);
  const ownStack = ArrayPrototypeSlice(
    StringPrototypeSplit(own.stack, '\n'), 1);

  const { 0: len, 1: off } = identicalSequenceRange(ownStack, errStack);
  if (len > 0) {
    ArrayPrototypeSplice(ownStack, off + 1, len - 2,
                         '    [... lines matching original stack trace ...]');
  }

  return err.stack + sep + ArrayPrototypeJoin(ownStack, '\n');
}

/**
 * Synchronously calls each of the listeners registered
 * for the event.
 * @param {string | symbol} type
 * @param {...any} [args]
 * @returns {boolean}
 */
EventEmitter.prototype.emit = function emit(type, ...args) {
  let doError = (type === 'error');

  const events = this._events;
  if (events !== undefined) {
    if (doError && events[kErrorMonitor] !== undefined)
      this.emit(kErrorMonitor, ...args);
    doError = (doError && events.error === undefined);
  } else if (!doError)
    return false;

  // 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, {
          value: FunctionPrototypeBind(enhanceStackTrace, this, er, capture),
          configurable: true
        });
      } catch {}

      // 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;
    const { inspect } = require('internal/util/inspect');
    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 handler = events[type];

  if (handler === undefined)
    return false;

  if (typeof handler === 'function') {
    const result = handler.apply(this, args);

    // We check if result is undefined first because that
    // is the most common case so we do not pay any perf
    // penalty
    if (result !== undefined && result !== null) {
      addCatch(this, result, type, args);
    }
  } else {
    const len = handler.length;
    const listeners = arrayClone(handler);
    for (let i = 0; i < len; ++i) {
      const result = listeners[i].apply(this, args);

      // We check if result is undefined first because that
      // is the most common case so we do not pay any perf
      // penalty.
      // This code is duplicated because extracting it away
      // would make it non-inlineable.
      if (result !== undefined && result !== null) {
        addCatch(this, result, type, args);
      }
    }
  }

  return true;
};

function _addListener(target, type, listener, prepend) {
  let m;
  let events;
  let existing;

  checkListener(listener);

  events = target._events;
  if (events === undefined) {
    events = target._events = ObjectCreate(null);
    target._eventsCount = 0;
  } else {
    // To avoid recursion in the case that type === "newListener"! Before
    // adding it to the listeners, first emit "newListener".
    if (events.newListener !== undefined) {
      target.emit('newListener', type,
                  listener.listener ?? listener);

      // Re-assign `events` because a newListener handler could have caused the
      // this._events to be assigned to a new object
      events = target._events;
    }
    existing = events[type];
  }

  if (existing === undefined) {
    // Optimize the case of one listener. Don't need the extra array object.
    events[type] = listener;
    ++target._eventsCount;
  } else {
    if (typeof existing === 'function') {
      // Adding the second element, need to change to array.
      existing = events[type] =
        prepend ? [listener, existing] : [existing, listener];
      // If we've already got an array, just append.
    } else if (prepend) {
      existing.unshift(listener);
    } else {
      existing.push(listener);
    }

    // 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
      // eslint-disable-next-line no-restricted-syntax
      const w = new Error('Possible EventEmitter memory leak detected. ' +
                          `${existing.length} ${String(type)} listeners ` +
                          `added to ${inspect(target, { depth: -1 })}. Use ` +
                          'emitter.setMaxListeners() to increase limit');
      w.name = 'MaxListenersExceededWarning';
      w.emitter = target;
      w.type = type;
      w.count = existing.length;
      process.emitWarning(w);
    }
  }

  return target;
}

/**
 * Adds a listener to the event emitter.
 * @param {string | symbol} type
 * @param {Function} listener
 * @returns {EventEmitter}
 */
EventEmitter.prototype.addListener = function addListener(type, listener) {
  return _addListener(this, type, listener, false);
};

EventEmitter.prototype.on = EventEmitter.prototype.addListener;

/**
 * Adds the `listener` function to the beginning of
 * the listeners array.
 * @param {string | symbol} type
 * @param {Function} listener
 * @returns {EventEmitter}
 */
EventEmitter.prototype.prependListener =
    function prependListener(type, listener) {
      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 this.listener.apply(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;
}

/**
 * Adds a one-time `listener` function to the event emitter.
 * @param {string | symbol} type
 * @param {Function} listener
 * @returns {EventEmitter}
 */
EventEmitter.prototype.once = function once(type, listener) {
  checkListener(listener);

  this.on(type, _onceWrap(this, type, listener));
  return this;
};

/**
 * Adds a one-time `listener` function to the beginning of
 * the listeners array.
 * @param {string | symbol} type
 * @param {Function} listener
 * @returns {EventEmitter}
 */
EventEmitter.prototype.prependOnceListener =
    function prependOnceListener(type, listener) {
      checkListener(listener);

      this.prependListener(type, _onceWrap(this, type, listener));
      return this;
    };

/**
 * Removes the specified `listener` from the listeners array.
 * @param {string | symbol} type
 * @param {Function} listener
 * @returns {EventEmitter}
 */
EventEmitter.prototype.removeListener =
    function removeListener(type, listener) {
      checkListener(listener);

      const events = this._events;
      if (events === undefined)
        return this;

      const list = events[type];
      if (list === undefined)
        return this;

      if (list === listener || list.listener === listener) {
        if (--this._eventsCount === 0)
          this._events = ObjectCreate(null);
        else {
          delete events[type];
          if (events.removeListener)
            this.emit('removeListener', type, list.listener || listener);
        }
      } else if (typeof list !== 'function') {
        let position = -1;

        for (let i = list.length - 1; i >= 0; i--) {
          if (list[i] === listener || list[i].listener === listener) {
            position = i;
            break;
          }
        }

        if (position < 0)
          return this;

        if (position === 0)
          list.shift();
        else {
          if (spliceOne === undefined)
            spliceOne = require('internal/util').spliceOne;
          spliceOne(list, position);
        }

        if (list.length === 1)
          events[type] = list[0];

        if (events.removeListener !== undefined)
          this.emit('removeListener', type, listener);
      }

      return this;
    };

EventEmitter.prototype.off = EventEmitter.prototype.removeListener;

/**
 * Removes all listeners from the event emitter. (Only
 * removes listeners for a specific event name if specified
 * as `type`).
 * @param {string | symbol} [type]
 * @returns {EventEmitter}
 */
EventEmitter.prototype.removeAllListeners =
    function removeAllListeners(type) {
      const events = this._events;
      if (events === undefined)
        return this;

      // Not listening for removeListener, no need to emit
      if (events.removeListener === undefined) {
        if (arguments.length === 0) {
          this._events = ObjectCreate(null);
          this._eventsCount = 0;
        } else if (events[type] !== undefined) {
          if (--this._eventsCount === 0)
            this._events = ObjectCreate(null);
          else
            delete events[type];
        }
        return this;
      }

      // Emit removeListener for all listeners on all events
      if (arguments.length === 0) {
        for (const key of ReflectOwnKeys(events)) {
          if (key === 'removeListener') continue;
          this.removeAllListeners(key);
        }
        this.removeAllListeners('removeListener');
        this._events = ObjectCreate(null);
        this._eventsCount = 0;
        return this;
      }

      const listeners = events[type];

      if (typeof listeners === 'function') {
        this.removeListener(type, listeners);
      } else if (listeners !== undefined) {
        // LIFO order
        for (let i = listeners.length - 1; i >= 0; i--) {
          this.removeListener(type, listeners[i]);
        }
      }

      return this;
    };

function _listeners(target, type, unwrap) {
  const events = target._events;

  if (events === undefined)
    return [];

  const evlistener = events[type];
  if (evlistener === undefined)
    return [];

  if (typeof evlistener === 'function')
    return unwrap ? [evlistener.listener || evlistener] : [evlistener];

  return unwrap ?
    unwrapListeners(evlistener) : arrayClone(evlistener);
}

/**
 * Returns a copy of the array of listeners for the event name
 * specified as `type`.
 * @param {string | symbol} type
 * @returns {Function[]}
 */
EventEmitter.prototype.listeners = function listeners(type) {
  return _listeners(this, type, true);
};

/**
 * Returns a copy of the array of listeners and wrappers for
 * the event name specified as `type`.
 * @param {string | symbol} type
 * @returns {Function[]}
 */
EventEmitter.prototype.rawListeners = function rawListeners(type) {
  return _listeners(this, type, false);
};

/**
 * Returns the number of listeners listening to the event name
 * specified as `type`.
 * @deprecated since v3.2.0
 * @param {EventEmitter} emitter
 * @param {string | symbol} type
 * @returns {number}
 */
EventEmitter.listenerCount = function(emitter, type) {
  if (typeof emitter.listenerCount === 'function') {
    return emitter.listenerCount(type);
  }
  return FunctionPrototypeCall(listenerCount, emitter, type);
};

EventEmitter.prototype.listenerCount = listenerCount;

/**
 * Returns the number of listeners listening to event name
 * specified as `type`.
 * @param {string | symbol} type
 * @returns {number}
 */
function listenerCount(type) {
  const events = this._events;

  if (events !== undefined) {
    const evlistener = events[type];

    if (typeof evlistener === 'function') {
      return 1;
    } else if (evlistener !== undefined) {
      return evlistener.length;
    }
  }

  return 0;
}

/**
 * Returns an array listing the events for which
 * the emitter has registered listeners.
 * @returns {any[]}
 */
EventEmitter.prototype.eventNames = function eventNames() {
  return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
};

function arrayClone(arr) {
  // At least since V8 8.3, this implementation is faster than the previous
  // which always used a simple for-loop
  switch (arr.length) {
    case 2: return [arr[0], arr[1]];
    case 3: return [arr[0], arr[1], arr[2]];
    case 4: return [arr[0], arr[1], arr[2], arr[3]];
    case 5: return [arr[0], arr[1], arr[2], arr[3], arr[4]];
    case 6: return [arr[0], arr[1], arr[2], arr[3], arr[4], arr[5]];
  }
  return ArrayPrototypeSlice(arr);
}

function unwrapListeners(arr) {
  const ret = arrayClone(arr);
  for (let i = 0; i < ret.length; ++i) {
    const orig = ret[i].listener;
    if (typeof orig === 'function')
      ret[i] = orig;
  }
  return ret;
}

/**
 * Returns a copy of the array of listeners for the event name
 * specified as `type`.
 * @param {EventEmitter | EventTarget} emitterOrTarget
 * @param {string | symbol} type
 * @returns {Function[]}
 */
function getEventListeners(emitterOrTarget, type) {
  // First check if EventEmitter
  if (typeof emitterOrTarget.listeners === 'function') {
    return emitterOrTarget.listeners(type);
  }
  // Require event target lazily to avoid always loading it
  const { isEventTarget, kEvents } = require('internal/event_target');
  if (isEventTarget(emitterOrTarget)) {
    const root = emitterOrTarget[kEvents].get(type);
    const listeners = [];
    let handler = root?.next;
    while (handler?.listener !== undefined) {
      const listener = handler.listener?.deref ?
        handler.listener.deref() : handler.listener;
      listeners.push(listener);
      handler = handler.next;
    }
    return listeners;
  }
  throw new ERR_INVALID_ARG_TYPE('emitter',
                                 ['EventEmitter', 'EventTarget'],
                                 emitterOrTarget);
}

/**
 * Creates a `Promise` that is fulfilled when the emitter
 * emits the given event.
 * @param {EventEmitter} emitter
 * @param {string} name
 * @param {{ signal: AbortSignal; }} [options]
 * @returns {Promise}
 */
async function once(emitter, name, options = {}) {
  const signal = options?.signal;
  validateAbortSignal(signal, 'options.signal');
  if (signal?.aborted)
    throw new AbortError();
  return new Promise((resolve, reject) => {
    const errorListener = (err) => {
      emitter.removeListener(name, resolver);
      if (signal != null) {
        eventTargetAgnosticRemoveListener(signal, 'abort', abortListener);
      }
      reject(err);
    };
    const resolver = (...args) => {
      if (typeof emitter.removeListener === 'function') {
        emitter.removeListener('error', errorListener);
      }
      if (signal != null) {
        eventTargetAgnosticRemoveListener(signal, 'abort', abortListener);
      }
      resolve(args);
    };
    eventTargetAgnosticAddListener(emitter, name, resolver, { once: true });
    if (name !== 'error' && typeof emitter.once === 'function') {
      emitter.once('error', errorListener);
    }
    function abortListener() {
      eventTargetAgnosticRemoveListener(emitter, name, resolver);
      eventTargetAgnosticRemoveListener(emitter, 'error', errorListener);
      reject(new AbortError());
    }
    if (signal != null) {
      eventTargetAgnosticAddListener(
        signal, 'abort', abortListener, { once: true });
    }
  });
}

const AsyncIteratorPrototype = ObjectGetPrototypeOf(
  ObjectGetPrototypeOf(async function* () {}).prototype);

function createIterResult(value, done) {
  return { value, done };
}

function eventTargetAgnosticRemoveListener(emitter, name, listener, flags) {
  if (typeof emitter.removeListener === 'function') {
    emitter.removeListener(name, listener);
  } else if (typeof emitter.removeEventListener === 'function') {
    emitter.removeEventListener(name, listener, flags);
  } else {
    throw new ERR_INVALID_ARG_TYPE('emitter', 'EventEmitter', emitter);
  }
}

function eventTargetAgnosticAddListener(emitter, name, listener, flags) {
  if (typeof emitter.on === 'function') {
    if (flags?.once) {
      emitter.once(name, listener);
    } else {
      emitter.on(name, listener);
    }
  } else if (typeof emitter.addEventListener === 'function') {
    // EventTarget does not have `error` event semantics like Node
    // EventEmitters, we do not listen to `error` events here.
    emitter.addEventListener(name, (arg) => { listener(arg); }, flags);
  } else {
    throw new ERR_INVALID_ARG_TYPE('emitter', 'EventEmitter', emitter);
  }
}

/**
 * Returns an `AsyncIterator` that iterates `event` events.
 * @param {EventEmitter} emitter
 * @param {string | symbol} event
 * @param {{ signal: AbortSignal; }} [options]
 * @returns {AsyncIterator}
 */
function on(emitter, event, options) {
  const signal = options?.signal;
  validateAbortSignal(signal, 'options.signal');
  if (signal?.aborted)
    throw new AbortError();

  const unconsumedEvents = [];
  const unconsumedPromises = [];
  let error = null;
  let finished = false;

  const iterator = ObjectSetPrototypeOf({
    next() {
      // First, we consume all unread events
      const value = unconsumedEvents.shift();
      if (value) {
        return PromiseResolve(createIterResult(value, false));
      }

      // Then we error, if an error happened
      // This happens one time if at all, because after 'error'
      // we stop listening
      if (error) {
        const p = PromiseReject(error);
        // Only the first element errors
        error = null;
        return p;
      }

      // If the iterator is finished, resolve to done
      if (finished) {
        return PromiseResolve(createIterResult(undefined, true));
      }

      // Wait until an event happens
      return new Promise(function(resolve, reject) {
        unconsumedPromises.push({ resolve, reject });
      });
    },

    return() {
      eventTargetAgnosticRemoveListener(emitter, event, eventHandler);
      eventTargetAgnosticRemoveListener(emitter, 'error', errorHandler);

      if (signal) {
        eventTargetAgnosticRemoveListener(
          signal,
          'abort',
          abortListener,
          { once: true });
      }

      finished = true;

      for (const promise of unconsumedPromises) {
        promise.resolve(createIterResult(undefined, true));
      }

      return PromiseResolve(createIterResult(undefined, true));
    },

    throw(err) {
      if (!err || !(err instanceof Error)) {
        throw new ERR_INVALID_ARG_TYPE('EventEmitter.AsyncIterator',
                                       'Error', err);
      }
      error = err;
      eventTargetAgnosticRemoveListener(emitter, event, eventHandler);
      eventTargetAgnosticRemoveListener(emitter, 'error', errorHandler);
    },

    [SymbolAsyncIterator]() {
      return this;
    }
  }, AsyncIteratorPrototype);

  eventTargetAgnosticAddListener(emitter, event, eventHandler);
  if (event !== 'error' && typeof emitter.on === 'function') {
    emitter.on('error', errorHandler);
  }

  if (signal) {
    eventTargetAgnosticAddListener(
      signal,
      'abort',
      abortListener,
      { once: true });
  }

  return iterator;

  function abortListener() {
    errorHandler(new AbortError());
  }

  function eventHandler(...args) {
    const promise = ArrayPrototypeShift(unconsumedPromises);
    if (promise) {
      promise.resolve(createIterResult(args, false));
    } else {
      unconsumedEvents.push(args);
    }
  }

  function errorHandler(err) {
    finished = true;

    const toError = ArrayPrototypeShift(unconsumedPromises);

    if (toError) {
      toError.reject(err);
    } else {
      // The next time we call next()
      error = err;
    }

    iterator.return();
  }
}