https://twitter.com/rauchg/status/273126592786608130

lib_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.

// ....

EventEmitter.prototype.addListener = function(type, listener, returnHandle) {
  if ('function' !== typeof listener) {
    throw new Error('addListener only takes instances of Function');
  }

  if (!this._events) this._events = {};

  // To avoid recursion in the case that type == "newListeners"! Before
  // adding it to the listeners, first emit "newListeners".
  if (this._events.newListener) {
    this.emit('newListener', type, typeof listener.listener === 'function' ?
              listener.listener : listener);
  }

  if (!this._events[type]) {
    // Optimize the case of one listener. Don't need the extra array object.
    this._events[type] = listener;
  } else if (isArray(this._events[type])) {

    // If we've already got an array, just append.
    this._events[type].push(listener);

  } else {
    // Adding the second element, need to change to array.
    this._events[type] = [this._events[type], listener];

  }

  // Check for listener leak
  if (isArray(this._events[type]) && !this._events[type].warned) {
    var m;
    if (this._maxListeners !== undefined) {
      m = this._maxListeners;
    } else {
      m = defaultMaxListeners;
    }

    if (m && m > 0 && this._events[type].length > m) {
      this._events[type].warned = true;
      console.error('(node) warning: possible EventEmitter memory ' +
                    'leak detected. %d listeners added. ' +
                    'Use emitter.setMaxListeners() to increase limit.',
                    this._events[type].length);
      console.trace();
    }
  }
  
  if(returnHandle) {
    return {
      destroy: this.removeListener.bind(this, type, listener)
    };
  }
  return this;
};

// ....