Tiny Observer

main.ts

import {Observer} from './core/event-emitter'

const observer = new Observer<string>()

const sub1$ = observer.subscribe((value, count) => {
  console.log('sub 1: ', value, count)
})

const sub2$ = observer.subscribe((value, count) => {
  console.log('sub 2: ', value, count)
})

observer.next('um')
observer.next('dois')

sub1$.unsubscribe()

observer.next('três')
observer.next('quatro')
observer.next('cinco')

sub2$.unsubscribe()

observer.next('seis')

observer.ts

export interface Observed {
  unsubscribe(): void
}

export interface Subscribe<T> {
  (value: T, count: number): void
}

export class Observer<T> {
  #count = 0
  #subject = new Set<Subscribe<T>>([])

  subscribe(cb: Subscribe<T>): Observed {
    this.#subject.add(cb)

    const unsubscribe = () => {
      this.#subject.delete(cb)
    }

    return {unsubscribe}
  }

  next(value: T) {
    this.#count++
    for (const cb of this.#subject) {
      cb(value, this.#count)
    }
  }
}