@herbcaudill/eventemitter42

EventEmitter42

This is a Typescript/ESM adaptation of eventemitter3.

The motivation behind this is that I often use a pattern of awaiting for a promisified event, and neither me nor the internet could get strong typing on the result of that promise.

type TestEvents = {
  foo: (payload: { bar: string }) => void
}

const testEmitter = new EventEmitter<TestEvents>()

const { bar } = await eventPromise(testEmitter, 'foo')
// error: Property 'bar' does not exist on type 'unknown'.

As a bonus this library includes that eventPromise function.

const { bar } = await eventPromise(testEmitter, 'foo')
// ✅ bar is `string`

Differences

I left out some eventemitter3 features that I've never used:

• Prefixed event names
• Passing context to a listener
• eventNames, listeners and listenerCount methods

With eventemitter3, you have various options for defining event listener signatures. In this library, event types can only be expressed as a map of function signatures:

const testEmitter = new EventEmitter<{
  foo: (p: string) => void
  bar: (p: number) => void
}>()

// won't work:

const no = new EventEmitter<'data'>() ❌
const nope = new EventEmitter<{foo: string; bar: number}>() ❌

Installation

$ pnpm add @herbcaudill/eventemitter42

Usage

import { EventEmitter, eventPromise } from '@herbcaudill/eventemitter42'

class MyEmitter extends EventEmitter<TestEvents> {
  doSomething() {
    this.emit('foo', 'here is a payload')
  }

  doSomethingElse() {
    this.emit('bar', 42)
  }
}

const emitter = new MyEmitter()

emitter.on('foo', p => {
  console.log(p) // 'here is a payload'
})

emitter.doSomething()

setTimeout(() => emitter.doSomethingElse(), 1000)

const result = await eventPromise(emitter, 'bar')
console.log(result) // '42' (after a second)