EventEmitter in node.js master, April 2017

eventsexcerpt.js

function EventEmitter() {
  EventEmitter.init.call(this);
}
/*...*/
EventEmitter.init = function() {
  this.domain = null;
  if (EventEmitter.usingDomains) {
    // if there is an active domain, then attach to it.
    domain = domain || require('domain');
    if (domain.active && !(this instanceof domain.Domain)) {
      this.domain = domain.active;
    }
  }

  if (!this._events || this._events === Object.getPrototypeOf(this)._events) {
    this._events = Object.create(null);
    this._eventsCount = 0;
  }
  this._maxListeners = this._maxListeners || undefined;
};