redux createStore returns [$$observable]: observable

observable.js

/**
this is from the Redux createStore()
  /**
   * Interoperability point for observable/reactive libraries.
   * @returns {observable} A minimal observable of state changes.
   * For more information, see the observable proposal:
   * https://github.com/tc39/proposal-observable
   */
  function observable() {
    const outerSubscribe = subscribe
    return {
      /**
       * The minimal observable subscription method.
       * @param {Object} observer Any object that can be used as an observer.
       * The observer object should have a `next` method.
       * @returns {subscription} An object with an `unsubscribe` method that can
       * be used to unsubscribe the observable from the store, and prevent further
       * emission of values from the observable.
       */
      subscribe(observer) {
        if (typeof observer !== 'object') {
          throw new TypeError('Expected the observer to be an object.')
        }

        function observeState() {
          if (observer.next) {
            observer.next(getState())
          }
        }

        observeState()
        const unsubscribe = outerSubscribe(observeState)
        return {
          unsubscribe
        }
      },

      [$$observable]() {
        return this
      }
    }
  }
*/
//we can use it like this:
import $$observable from 'symbol-observable'

const obj = {
    next: (state) => { // need to provide next method
        console.log(state)
    }
}

const observerOne = store[$$observable]().subscribe(obj)
observerOne.unsubscribe()