Rettime

A type-safe marriage of EventTarget and EventEmitter.

Features

• 🎯 Event-based. Control event flow: prevent defaults, stop propagation, cancel events. Something your common Emitter can't do.
• 🗼 Emitter-inspired. Emit event types and data, don't bother with creating Event instances. A bit less verbosity than a common EventTarget.
• ⛑️ Type-safe. Describe the exact event types and payloads accepted by the emitter. Never emit or listen to unknown events.
• 🧰 Convenience methods like .emitAsPromise() and .emitAsGenerator() to build more complex event-driven systems.
• 🐙 Tiny. 700B gzipped.

[!WARNING] This library does not have performance as the end goal. In fact, since it operates on events and supports event cancellation, it will likely be slower than the emitters that don't do that.

Motivation

Why not just EventTarget?

The EventTarget API is fantastic. It works in the browser and in Node.js, dispatches actual events, supports cancellation, etc. At the same time, it has a number of flaws that prevent me from using it for anything serious:

• Complete lack of type safety. The type in new Event(type) is not a type argument in lib.dom.ts. It's always string. It means it's impossible to narrow it down to a literal string type to achieve type safety.
• No concept of .prependListener(). There is no way to add a listener to run first, before other existing listeners.
• No concept of .removeAllListeners(). You have to remove each individual listener by hand. Good if you own the listeners, not so good if you don't.
• No concept of .listenerCount() or knowing if a dispatched event had any listeners (the boolean returned from .dispatch() indicates if the event has been prevented, not whether it had any listeners).
• (Opinionated) Verbose. I prefer .on() over .addEventListener(). I prefer passing data than constructing new MessageEvent() all the time.

Why not just Emitter (in Node.js)?

The Emitter API in Node.js is great, but it has its own downsides:

• Node.js-specific. Emitter does not work in the browser.
• Lacks any type safety.
• No concept of .stopPropagation() and .stopImmediatePropagation(). Those methods are defined but literally do nothing.

Install

npm install rettime

API

TypedEvent

TypedEvent is a subset of MessageEvent that allows for type-safe event declaration.

new TypedEvent<DataType, ReturnType, EventType>(type: EventType, { data: DataType })

The data argument depends on the DataType of your event. Use void if the event must not send any data.

Custom events

You can implement custom events by extending the default TypedEvent class and forwarding the type arguments that it expects:

class GreetingEvent<
  DataType = void,
  ReturnType = any,
  EventType extends string = string,
> extends TypedEvent<DataType, ReturnType, EventType> {
  public id: string
}

const emitter = new Emitter<{ greeting: GreetingEvent<'john'> }>()

emitter.on('greeting', (event) => {
  console.log(event instanceof GreetingEvent) // true
  console.log(event instanceof TypedEvent) // true
  console.log(event instanceof MessageEvent) // true

  console.log(event.type) // "greeting"
  console.log(event.data) // "john"
  console.log(event.id) // string
})

Emitter

new Emitter<EventMap>()

The EventMap type argument allows you describe the supported event types, their payload, and the return type of their event listeners.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ hello: TypedEvent<string, number> }>()

emitter.on('hello', () => 1) // ✅
emitter.on('hello', () => 'oops') // ❌ string not assignable to type number

emitter.emit(new TypedEvent('hello', { data: 'John' })) // ✅
emitter.emit(new TypedEvent('hello', { data: 123 })) // ❌ number is not assignable to type string
emitter.emit(new TypedEvent('hello')) // ❌ missing data argument of type string

emitter.emit(new TypedEvent('unknown')) // ❌ "unknown" does not satisfy "hello"

Describing events

The Emitter class requires a type argument that describes the event map. If you do not provide that argument, adding listeners or emitting events will produce a type error as your emitter doesn't have an event map defined.

An event map is an object of the following shape:

{
  [type: string]: TypedEvent
}

The type is a string indicating the event type (e.g. greet or ping). The array it accepts has two members: args describes the arguments accepted by this event (can also be never for events without arguments) and returnValue is an optional type for the data returned from the listeners for this event.

Let's say you want to define a greet event that expects a user name as data and returns a greeting string:

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ greet: TypedEvent<string, string> }>()

emitter.on('greet', (event) => {
  console.log(`Hello, ${event.data}!`)
})
emitter.emit(new TypedEvent('greet', { data: 'John' }))
// "Hello, John!"

Here's another example where we define a ping event that has no arguments but returns a timestamp for each ping:

const emitter = new Emitter<{ ping: TypedEvent<void, number> }>()
emitter.on('ping', () => Date.now())
const results = await emitter.emitAsPromise(new TypedEvent('ping'))
// [1745658424732]

[!IMPORTANT] When providing type arguments to your TypedEvents, you do not need to provide the EventType argument—it will be inferred from your event map.

.on(type, listener[, options])

Adds an event listener for the given event type.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ hello: TypedEvent<string> }>()

emitter.on('hello', (event) => {
  // `event` is a `TypedEvent` instance derived from `MessageEvent`.
  console.log(event.data)
})

All methods that add new listeners return an AbortController instance bound to that listener. You can use that controller to cancel the event handling, including mid-air:

const controller = emitter.on('hello', listener)
controller.abort(reason)

All methods that add new listeners also accept an optional options argument. You can use it to configure event handling behavior. For example, you can provide an existing AbortController signal as the options.signal value so the attached listener abides by your controller:

emitter.on('hello', listener, { signal: controller.signal })

Both the public controller of the event and your custom controller are combined using AbortSignal.any().

.once(type, listener[, options])

Adds a one-time event listener for the given event type.

.earlyOn(type, listener[, options])

Prepends a listener for the given event type.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ hello: TypedEvent<void, number> }>()

emitter.on('hello', () => 1)
emitter.earlyOn('hello', () => 2)

const results = await emitter.emitAsPromise(new TypedEvent('hello'))
// [2, 1]

.earlyOnce(type, listener[, options])

Prepends a one-time listener for the given event type.

.emit(type[, data])

Emits the given event with optional data.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ hello: TypedEvent<string> }>()

emitter.on('hello', (event) => console.log(event.data))

emitter.emit(new TypedEvent('hello', 'John'))

.emitAsPromise(type[, data])

Emits the given event and returns a Promise that resolves with the returned data of all matching event listeners, or rejects whenever any of the matching event listeners throws an error.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ hello: TypedEvent<number, Promise<number>> }>()

emitter.on('hello', async (event) => {
  await sleep(100)
  return event.data + 1
})
emitter.on('hello', async (event) => event.data + 2)

const values = await emitter.emitAsPromise(new TypedEvent('hello', { data: 1 }))
// [2, 3]

Unlike .emit(), the .emitAsPromise() method awaits asynchronous listeners.

.emitAsGenerator(type[, data])

Emits the given event and returns a generator function that exhausts all matching event listeners. Using a generator gives you granular control over what listeners are called.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ hello: TypedEvent<string, number> }>()

emitter.on('hello', () => 1)
emitter.on('hello', () => 2)

for (const listenerResult of emitter.emitAsGenerator(
  new TypedEvent('hello', { data: 'John' }),
)) {
  // Stop event emission if a listener returns a particular value.
  if (listenerResult === 1) {
    break
  }
}

.listeners([type])

Returns the list of all event listeners matching the given event type. If no event type is provided, returns the list of all existing event listeners.

.listenerCount([type])

Returns the number of the event listeners matching the given event type. If no event type is provided, returns the total number of existing listeners.

.removeListener(type, listener)

Removes the event listener for the given event type.

.removeAllListeners([type])

Removes all event listeners for the given event type. If no event type is provided, removes all existing event listeners.

Types

This library also comes with a set of helper types to make your life easier.

Emitter.EventType

Returns the Event type (or its subtype) representing the given listener.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ greeting: TypedEvent<'john'> }>()
type GreetingEvent = Emitter.EventType<typeof emitter, 'greeting'>
// TypedEvent<'john'>

Emitter.ListenerType

Returns the type of the given event's listener.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ greeting: TypedEvent<string, number[]> }>()
type GreetingListener = Emitter.ListenerType<typeof emitter, 'greeting'>
// (event: TypedEvent<string>) => number[]

The ListenerType helper is in itself type-safe, allowing only known event types as the second argument.

Emitter.ListenerReturnType

Returns the return type of the given event's listener.

import { Emitter, TypedEvent } from 'rettime'

const emitter = new Emitter<{ getTotalPrice: TypedEvent<Cart, number> }>()
type CartTotal = Emitter.ListenerReturnType<typeof emitter, 'getTotalPrice'>
// number