Emitter that works with generator methods using co

Usage no npm install needed!

<script type="module">
  import coEmitter from '';



co-powered emitter mixin for objects Build Status


npm install co-emitter



The Emitter may be used as a mixin or a standalone object.


var Emitter = require('co-emitter');
var emitter = new Emitter();

emitter.on('say', function *(message) {
  yield delay(200);
  console.log("I say: " + message);
  return true;

co(function *() {
  var result = yield emitter.emit('say', 'hello')
// result == true, and console.logs "I say: hello""


var user = { name: 'Tobi' };
yield user.emit('some event');

Prototype mixin:

var User = function() { = 'Tobi';


var user = new User();
yield user.emit('some event');

Emitter#on(name, fn*, [...moreFns*])

Registers generator as a listener for name.

fn* can be:

  • a generator function
  • an array of multiple generator functions (will be run in the order passed in)
var emitter = new Emitter();
var listenerA = function *() {},
    listenerB = function *() {},
    listenerC = function *() {};

// All of these would be equal

emitter.on('test', listenerA);
emitter.on('test', listenerB);
emitter.on('test', listenerC);

// or

emitter.on('test', [listenerA, listenerB, listenerC]);

// or

emitter.on('test', listenerA, listenerB, listenerC);

Emitter#once(name, fn*, [...moreFns*])

Registers generator as a listener for name. This generator runs once and is then removed.

var emitter = new Emitter();
emitter.once('test', function*() { console.log("Yay!"); });
yield emitter.emit('test');

Emitter#emit(event, ...args)

Runs the registered listeners for event. Passes in optional args to the listeners.

args can be an array, or multiple arguments.

Returns the result of the last run listener. If results are omitted, returns original arguments.

var result = yield emitter.emit('add', 2, 2);
console.log(result) // 4
Async and Sync Sync Events

If you want an event to be synchronous, and not have to yield to emitter.emit, you can do so by using normal functions for your listeners.

var callCount = 0;
emitter.on('call', function() {


If you wan't to wait for async operations to have finished, you may do so by using yield within generators.

emitter.on('saving', function *() {
  userStatistics.saveCount += 1;

// Elsewhere...
yield emitter.emit('saving'); // wait for `saving`'s async to have finished.

You may not use both generators and normal functions as listeners for a single event. If you need async and sync for a single event, use a non-yielding generator for the sync.

emitter.on('saving', function*() {
  userStatistics.saveCount += 1;

emitter.on('saving', function *() {

// Elsewhere...
yield emitter.emit('saving'); // wait for `saving`'s async to have finished.

Because we are using one async function (or a generator), we must use yield when calling emit.

Emitter chaining

Listeners run with a middleware pattern, results from one listener can be piped into the next. For example:

var emitter = new Emitter();

var uppercase = function *(str) {
  return str.toUpperCase();

var firstChar = function *(str) {
  return str.charAt(0)

emitter.on('upperFirst', uppercase, firstChar);

var result = yield emitter.emit('upperFirst', 'hello world');
// result == 'H'

If you do not return a value, it will use the initial arguments passed in/ the value of the last returning listener.

Emitter#off([event], [listener])

Removes a given listener for a specific listener.

  • If no event is provided, all listeners will be removed.
  • If no listener is provided, all listeners for given event will be removed.
  • If event and listener provided, removes listener from event


Convenience method to call off with no event or listener, thereby removing all listeners.


Returns whether or not any listeners exist for event.


Returns an array of all listeners for a given event. If no listeners exist, returns an empty array.