EventEmitter.js

function EventEmitter(options) {
  this._listeners = []
}

EventEmitter.prototype.on = function(event, handler) {
  this._listeners[event] = this._listeners[event] || []
  this._listeners[event].push(handler)
}

EventEmitter.prototype.off = function(event, handler) {
  if (this._listeners[event]) {
    this._listeners[event] = this._listeners[event].filter(h => h !== handler)
  }
}

EventEmitter.prototype.emit = function(event, data) {
  if (this._listeners[event]) {
    this._listeners[event].forEach(handler => {
      handler({ event, data })
    })
  }
}