Redux-like store implemented in observables

createStore.js

import xs from 'xstream';

// If I intended to make a serious state-management system based on observables, I would probably not try to imitate Redux 
// but this was made only for self-learning and for fun.

// Redux-like store factory:

const createStore = (reducer, initial) => {
    const action$ = xs.create({start() {},stop() {}});

    // this one line is core Redux functionality:
    const state$ = action$.fold(reducer, initial);

    let lastState = initial;
    state$.subscribe({
        next(state) {
            lastState = state
        }
    });
    return {
        dispatch: action$.shamefullySendNext.bind(action$),
        getState: () => lastState,
        subscribe: f => {
            state$.subscribe({next: f});
        },
    }
};

// ----------------------------------------------------------------------
//  Counter hello world:
// -----------------------------------------------------------------------

const counter = (state, action)  => {
    switch (action.type) {
        case 'inc':
            return {counter: state.counter + 1}
        case 'dec':
            return {counter: state.counter - 1}

        default:
            return state;
    }
};

const store = createStore(counter, {counter: 1});

store.subscribe(() => {
    console.log("UPDATE", store.getState())
});

store.dispatch({type: 'inc'});
store.dispatch({type: 'inc'});
store.dispatch({type: 'inc'});
store.dispatch({type: 'dec'});