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

firebaseChannel.test.js

import { eventChannel } from 'redux-saga'

function firebaseChannel(firebase, {
  eventType = 'value',
  returnSnapshot = false,
} = {}) {
  return eventChannel(emit => {
    const subscription = firebase.on(eventType, snapshot => {
      emit(returnSnapshot ? { snapshot } : { value: snapshot.val() })
    })
    return () => firebase.off(eventType, subscription)
  })
}

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

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

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

  emit({ type: t, snapshot }) {
    this.subs.forEach(({ type, listener }) => {
      if (type === t) listener(snapshot)
    })
  }
}

class FakeSnapshot {
  val = jest.fn(() => 42)
}

describe('firebaseChannel', () => {
  it('should subscribe to firebase, emit the values into the channel that firebase ' +
  'emits, and unsubscribe when the channel is closed', async () => {
    const firebase = new FakeFirebase()
    const channel = firebaseChannel(firebase)
    const taken = new Promise(resolve => channel.take(resolve))
    const snapshot = new FakeSnapshot()
    firebase.emit({ type: 'value', snapshot })
    const value = await taken
    expect(value).toEqual({ value: snapshot.val() })

    const spy = jest.spyOn(firebase, 'off')
    channel.close()
    expect(spy).toHaveBeenCalledWith('value', 1)
  })
})