pwfoo / nanoevents

Simple and tiny (72 bytes) event emitter library for JavaScript

Nano Events

Simple and tiny event emitter library for JavaScript.

• Only 97 bytes (minified and gzipped). It uses Size Limit to control size.
• on method returns unbind function. You don’t need to save callback to variable for removeListener.
• TypeScript and ES modules support.
• No aliases, just emit and on methods. No Node.js EventEmitter compatibility.

import { createNanoEvents } from 'nanoevents'

const emitter = createNanoEvents()

const unbind = emitter.on('tick', volume => {
  summary += volume
})

emitter.emit('tick', 2)
summary //=> 2

unbind()
emitter.emit('tick', 2)
summary //=> 2

Table of Contents

• Install
• TypeScript
• Mixing to Object
• Add Listener
• Remove Listener
• Execute Listeners
• Events List
• Once
• Remove All Listeners

Install

npm install nanoevents

TypeScript

Nano Events accepts interface with event name to listener argument types mapping.

interface Events {
  set: (name: string, count: number) => void,
  tick: () => void
}

const emitter = createNanoEvents<Events>()

// Correct calls:
emitter.emit('set', 'prop', 1)
emitter.emit('tick')

// Compilation errors:
emitter.emit('set', 'prop', '1')
emitter.emit('tick', 2)

Mixing to Object

Because Nano Events API has only just 2 methods, you could just create proxy methods in your class or encapsulate them entirely.

class Ticker {
  constructor () {
    this.emitter = createNanoEvents()
    this.internal = setInterval(() => {
      this.emitter.emit('tick')
    }, 100)
  }

  stop () {
    clearInterval(this.internal)
    this.emitter.emit('stop')
  }

  on (event, callback) {
    return this.emitter.on(event, callback)
  }
}

With Typescript:

import { createNanoEvents, Emitter } from "nanoevents"

interface Events {
  start: (startedAt: number) => void
}

class Ticker {
  emitter: Emitter

  constructor () {
    this.emitter = createNanoEvents<Events>()
  }

  on<E extends keyof Events>(event: E, callback: Events[E]) {
    return this.emitter.on(event, callback)
  }
}

Add Listener

Use on method to add listener for specific event:

emitter.on('tick', number => {
  console.log(number)
})

emitter.emit('tick', 1)
// Prints 1
emitter.emit('tick', 5)
// Prints 5

In case of your listener relies on some particular context (if it uses this within itself) you have to bind required context explicitly before passing function in as a callback.

var app = {
  userId: 1,
  getListener () {
    return () => {
      console.log(this.userId)
    }
  }
}
emitter.on('print', app.getListener())

Note: binding with use of the .bind() method won’t work as you might expect and therefore is not recommended.

Remove Listener

Methods on returns unbind function. Call it and this listener will be removed from event.

const unbind = emitter.on('tick', number => {
  console.log('on ' + number)
})

emitter.emit('tick', 1)
// Prints "on 1"

unbind()
emitter.emit('tick', 2)
// Prints nothing

Execute Listeners

Method emit will execute all listeners. First argument is event name, others will be passed to listeners.

emitter.on('tick', (a, b) => {
  console.log(a, b)
})
emitter.emit('tick', 1, 'one')
// Prints 1, 'one'

Events List

You can get used events list by events property.

const unbind = emitter.on('tick', () => { })
emitter.events //=> { tick: [ [Function] ] }

Once

If you need add event listener only for first event dispatch, you can use this snippet:

class Ticker {
  constructor () {
    this.emitter = createNanoEvents()
  }
  …
  once (event, callback) {
    const unbind = this.emitter.on(event, (...args) => {
      unbind()
      callback(...args)
    })
    return unbind
  }
}

Remove All Listeners

emitter.on('event1', () => { })
emitter.on('event2', () => { })

emitter.events = { }