Generic EventEmitter

EventEmitter.ts

class Emitter<Events extends Record<any, (val: any) => any>> {
  private observers: Map<keyof Events, Set<(val: any) => any>> = new Map()

  on<Name extends keyof Events, Fn extends Events[Name]>(name: Name, fn: Fn) {
    if (!this.observers.has(name)) {
      this.observers.set(name, new Set())
    }

    this.observers.get(name)!.add(fn)

    return () => this.off(name, fn)
  }

  off<Name extends keyof Events, Fn extends Events[Name]>(name: Name, fn: Fn) {
    this.observers.get(name)?.delete(fn)
  }

  emit<Name extends keyof Events, Val extends Parameters<Events[Name]>[0]>(
    name: Name,
    val: Val
  ) {
    if (!this.observers.has(name)) {
      return
    }

    for (const fn of this.observers.get(name)!) {
      fn(val)
    }
  }
}

// Support for anything as an event name
const exit = Symbol()

const emitter = new Emitter<{
  shoutName: (username: string) => void
  sayNextAge: (age: number) => void
  [exit]: (code: number) => void
}>()

emitter.on('shoutName', (name) => console.log(`Hi ${name.toUpperCase()}!`))
emitter.on('sayNextAge', (age) =>
  console.log(`You are soon gonna be ${age + 1} years old!`)
)

const unsub = emitter.on(exit, (code) => process.exit(code))
unsub() // on returns an unsubscribe function

emitter.emit('shoutName', 'Christophe')
emitter.emit('sayNextAge', 31)
emitter.emit(exit, 1)