Typed Event Emitter

A string-keyed event bus is quick to write and just as quick to get wrong - emit('user:login', {}) with the wrong shape fails silently. Anchor the whole thing to one Record<eventName, payload> map instead, and on / emit type-check against it, with the payload narrowed per event.

@itsnas usage.ts

codeusage.ts

type AppEvents = {
  'user:login': { userId: string }
  'cart:add': { sku: string; quantity: number }
  toast: string
}
 
// @src/code/emitter
const bus = new Emitter<AppEvents>()
 
const stop = bus.on('user:login', ({ userId }) => {
  identify(userId) // payload is { userId: string }
})
 
bus.emit('cart:add', { sku: 'A-100', quantity: 2 })
 
bus.emit('cart:add', { sku: 'A-100' }) // ✗ missing `quantity`
bus.emit('user:logout', undefined) // ✗ not a key of AppEvents
 
stop() // unsubscribe

main

Nas (@itsnas)

type Handler<T> = (payload: T) => void
 
export class Emitter<Events extends Record<string, unknown>> {
  // One handler Set per event. Held as `never` internally so the
  // heterogeneous map type-checks; the public methods re-narrow it.
  private handlers = new Map<keyof Events, Set<Handler<never>>>()
 
  on<K extends keyof Events>(
    event: K,
    handler: Handler<Events[K]>,
  ): () => void {
    const set = this.handlers.get(event) ?? new Set()
    set.add(handler as Handler<never>)
    this.handlers.set(event, set)
    return () => set.delete(handler as Handler<never>)
  }
 
  emit<K extends keyof Events>(event: K, payload: Events[K]): void {
    this.handlers
      .get(event)
      ?.forEach(handler => handler(payload as never))
  }
}

main

Nas (@itsnas)