Map based event emitter in ~350 bytes

Usage no npm install needed!

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



ES6 Map based event emitter in ~350 bytes

Build Status npm version npm dependencies

Arbitrary-emitter stores listeners in ES6 maps, so you can use any kind of value as key for your events

const emitter = arbitraryEmitter()
const key = {}
emitter.on(key, () => doSomething())
// will `doSomething`
  • ~350 bytes when gzipped
  • conventional api (on, off, once and emit)
  • check weak-emitter for a version that uses weakmaps to store events


Install with npm or yarn, clone the repo or download and extract the zip. Then import or insert it as script tag.

Emitter API

on(key, handler)

Adds the handler function to the event tagged with key. key can be any type of value. Every handler will be added once, despite the number of times it was added to the event. Handlers are invoked in the order they were added.

const key = {}
emitter.on(key, () => doSomething())
emitter.emit(key) // will `doSomething`

once(key, handler)

Same as on, but listener will be triggered just once, then it will be removed.

const key = {}
emitter.once(key, () => doSomethingOnce())
emitter.emit(key) // will `doSomethingOnce`
emitter.emit(key) // won't do anything

emit(key[, ...args])

Invoke all handlers tagged with key, passing the rest of the arguments

emitter.on('test', (opts) => console.log(opts.test))
const options = { test: 'Testing!' }
emitter.emit('test', options) // => 'Testing!'

off([key[, handler]])

  • If a key but no handler is passed the event will be removed
  • If key and handler are passed as arguments just the handler will be removed from the event, action) // will remove action from listeners // will remove all the listeners tagged with `key`, and the tag itself


Create and return an array with all the handlers for the event tagged with key

const f1 = () => alert('hi')
const f2 = () => alert('ho')
emitter.on('a', f1)
emitter.on('a', f2)

// ==> [f1, f2]


npm test

© 2020 Jacobo Tabernero - Released under MIT License