mitt

What is mitt?

The mitt npm package is a tiny, functional event emitter or pub/sub library that provides a way to emit and listen for events in JavaScript applications. It is designed to be simple, small, and fast, making it suitable for use in both the browser and Node.js environments.

What are mitt's main functionalities?

Event Emission

Emitting events with optional data. Listeners can subscribe to these events and react accordingly.

{"const mitt = require('mitt');\nconst emitter = mitt();\nemitter.emit('myEvent', { some: 'data' });"}

Event Listening

Subscribing to events. This allows functions to listen for specific events and execute code when those events are emitted.

{"const mitt = require('mitt');\nconst emitter = mitt();\nemitter.on('myEvent', (data) => {\n  console.log('Event received with data:', data);\n});"}

Event Unlistening

Removing event listeners. This provides a way to unsubscribe from events when a listener is no longer needed or the context changes.

{"const mitt = require('mitt');\nconst emitter = mitt();\nconst handler = (data) => {\n  console.log('Event received with data:', data);\n};\nemitter.on('myEvent', handler);\nemitter.off('myEvent', handler);"}

All Events Listening

Listening to all events. This special event type allows a listener to react to all events emitted by the emitter.

{"const mitt = require('mitt');\nconst emitter = mitt();\nemitter.on('*', (type, event) => {\n  console.log(`Type: ${type}, Event:`, event);\n});"}

Other packages similar to mitt

eventemitter3

EventEmitter3 is a high-performance event emitter. It is similar to mitt but provides more features like context binding and wildcard listeners. It is also slightly larger in size compared to mitt.

tiny-emitter

TinyEmitter is another small event emitter library that offers basic event emitting and listening functionality. It is comparable to mitt in terms of size and simplicity but does not support wildcard listeners.

events

The 'events' package is Node.js's EventEmitter class for the browser. It is more feature-rich than mitt, including asynchronous event emitting and more complex event handling capabilities, but it is also larger in size.

README

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+.

Table of Contents

• 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.

$ npm install --save mitt

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

// using ES6 modules
import mitt from 'mitt'

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

The UMD build is also available on unpkg:

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

You can find the library on window.mitt.

Usage

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.

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:

import mitt, { Emitter } from 'mitt';

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

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

Examples & Demos

Preact + Mitt Codepen Demo

---

API

Table of Contents

• 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

Register an event handler for the given type.

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.

Submitting pull requests

Pull requests are the greatest contributions, so be sure they are focused in scope, and do avoid unrelated commits.

• Fork it!
• Clone your fork: git clone https://github.com/<your-username>/mitt
• Navigate to the newly cloned directory: cd mitt
• Create a new branch for the new feature: git checkout -b my-new-feature
• Install the tools necessary for development: npm install
• Make your changes.
• Commit your changes: git commit -am 'Add some feature'
• Push to the branch: git push origin my-new-feature
• Submit a pull request with full remarks documenting your changes.

License

MIT License © Jason Miller