Mitt

Tiny 200b functional event emitter / pubsub.

Microscopic: weighs less than 200 bytes gzipped
Useful: a wildcard "*" event type listens to all events
Familiar: same names & ideas as Node's EventEmitter
Functional: methods don't rely on this
Great Name: somehow mitt wasn't taken

Mitt was made for the browser, but works in any JavaScript runtime. It has no dependencies and supports IE9+.

Install
Usage
Examples & Demos
API
Contribute
License

Install

This project uses node and npm. Go check them out if you don't have them locally installed.

sh
$ npm install --save mitt

Then with a module bundler like rollup or webpack, use as you would anything else:

javascript
// using ES6 modules
import mitt from 'mitt'

// using CommonJS modules
var mitt = require('mitt')

The UMD build is also available on unpkg:

html
<script src="https://unpkg.com/mitt/dist/mitt.umd.js"></script>

You can find the library on window.mitt.

Usage

js
import mitt from 'mitt'

const emitter = mitt()

// listen to an event
emitter.on('foo', e => console.log('foo', e) )

// listen to all events
emitter.on('*', (type, e) => console.log(type, e) )

// fire an event
emitter.emit('foo', { a: 'b' })

// clearing all events
emitter.all.clear()

// working with handler references:
function onFoo() {}
emitter.on('foo', onFoo)   // listen
emitter.off('foo', onFoo)  // unlisten

Typescript

Set "strict": true in your tsconfig.json to get improved type inference for mitt instance methods.

ts
import mitt from 'mitt';

type Events = {
  foo: string;
  bar?: number;
};

const emitter = mitt<Events>(); // inferred as Emitter<Events>

emitter.on('foo', (e) => {}); // 'e' has inferred type 'string'

emitter.emit('foo', 42); // Error: Argument of type 'number' is not assignable to parameter of type 'string'. (2345)

Alternatively, you can use the provided Emitter type:

ts
import mitt, { Emitter } from 'mitt';

type Events = {
  foo: string;
  bar?: number;
};

const emitter: Emitter<Events> = mitt<Events>();

Examples & Demos

<a href="http://codepen.io/developit/pen/rjMEwW?editors=0110"> Preact + Mitt Codepen Demo <img src="https://i.imgur.com/CjBgOfJ.png" width="278" alt="preact + mitt preview"> </a>

API

mitt
all
on
- Parameters
off
- Parameters
emit
- Parameters

mitt

Mitt: Tiny (~200b) functional event emitter / pubsub.

Returns Mitt

all

A Map of event names to registered handler functions.

on

Parameters

type (string | symbol) Type of event to listen for, or '*' for all events
handler Function Function to call in response to given event

off

Remove an event handler for the given type.
If handler is omitted, all handlers of the given type are removed.

Parameters

type (string | symbol) Type of event to unregister handler from, or '*'
handler Function? Handler function to remove

emit

Invoke all handlers for the given type.
If present, '*' handlers are invoked after type-matched handlers.

Note: Manually firing '*' handlers is not supported.

Parameters

type (string | symbol) The event type to invoke
evt Any? Any value (object is recommended and powerful), passed to each handler

Contribute

First off, thanks for taking the time to contribute!
Now, take a moment to be sure your contributions make sense to everyone else.

Reporting Issues

Found a problem? Want a new feature? First of all see if your issue or idea has already been reported.
If don't, just open a new clear and descriptive issue.