Example of making a channel in redux-saga with a generic emitter source and testing it with jest

emitterChannel.test.js

import { eventChannel } from 'redux-saga'

function emitterChannel(emitter, eventType) {
  return eventChannel(emit => {
    emitter.on(eventType, emit)
    return () => emitter.off(eventType, emit)
  })
}

describe('emitterChannel', () => {
  let emitter

  beforeEach(() => {
    emitter = new FakeEmitter()
  })

  it('should subscribe to the emitter, emit what it emits, and unsubscribe when closed', async () => {
    const type = 'bambino'
    const channel = emitterChannel(emitter, type)
    for (let i = 0; i < 3; i++) {
      const taken = new Promise(resolve => channel.take(resolve))
      const event = { type, value: i }
      emitter.emit(event)
      const e = await taken
      expect(e).toEqual(event)
    }

    emitter.off = jest.fn((eType, listener) => {
      expect(eType).toBe(type)
      expect(listener).toBeInstanceOf(Function)
    })
    channel.close()
    // Calling close multiple times only unsubscribes once
    channel.close()
    expect(emitter.off).toHaveBeenCalledTimes(1)
  })
})

class FakeEmitter {
  constructor() {
    this.subs = []
    this.count = 0
  }

  on(type, listener) {
    this.subs.push({
      type,
      listener,
      id: this.count++,
    })
  }

  off(type, listener) {
    this.subs = this.subs.filter(sub => sub.listener !== listener)
  }

  emit(event) {
    this.subs.forEach(({ type, listener }) => {
      if (type === event.type) listener(event)
    })
  }
}