Simple React/Redux example, taken from Dan Abramov's Redux tutorial series on Egghead.io

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import expect from 'expect';
import { createStore } from 'redux';

/**
 * The reducer function which takes a state and action,
 * returns the new state
 *
 * @param  {Number} state  old state from the store
 * @param  {Object} action action which is run to update the store
 * @return {Number}        updated store state
 */
const counter = (state = 0, action) => {
    switch (action.type) {
        case 'INCREMENT':
        return state + 1;
        case 'DECREMENT':
        return state - 1;
        default:
        return state;
    }
};

// A simple create store implementation similar to react
//
// /**
//  * Method takes a reducer function and returns an object
//  * which exposes getState, dispatch and subscribe methods
//  *
//  * @param  {Function} reducer    a reducer method which returns updated state
//  * @return {Object}   created    store object which contains getState, dispatch and subscribe methods
//  */
// const createStore = (reducer) => {
//     let state;            // holds the state
//     let listeners = [];   // holds reference to the added listeners
//
//     // method returns state
//     const getState = () => state;
//
//     // methods enables callback subscribing
//     // returns unsubscribe method
//     const subscribe = (listener) => {
//         listeners.push(listener);
//         return () => {
//             listeners = listeners.filter((l) => l !== listener);
//         };
//     };
//
//     // method triggers the reducer and listener update
//     const dispatch = (action) => {
//         state = reducer(state, action);
//         listeners.forEach(listener => listener());
//     };
//
//     // trigger dispatch once to set the state to default
//     dispatch({});
//
//     // expose the object w/ methods
//     return {getState, dispatch, subscribe};
// };

// create a store with counter reducer
const store = createStore(counter);

// Component which we'll render (Uses React internally)
const Counter = ({value, onIncrement, onDecrement}) => (
    <div>
        <h1>{value}</h1>
        <button onClick={onIncrement}>+</button>
        <button onClick={onDecrement}>-</button>
    </div>
);

// Render method, renders the Counter to the DOM
const render = () => {
    ReactDOM.render(
        <Counter value={store.getState()}
                 onIncrement={() =>
                    store.dispatch({
                        type: 'INCREMENT'
                    })
                 }
                 onDecrement={() =>
                    store.dispatch({
                        type: 'DECREMENT'
                    })
                 } />,
        document.getElementById('root')
    );
};

// subscribe to store changes
// set render as callback
store.subscribe(render);

// trigger it once to render initialy
render();

// added some tests for the store
expect(
    counter(0, {type: 'INCREMENT'})
).toEqual(1);

expect(
    counter(1, {type: 'INCREMENT'})
).toEqual(2);

expect(
    counter(2, {type: 'DECREMENT'})
).toEqual(1);

expect(
    counter(1, {type: 'DECREMENT'})
).toEqual(0);

expect(
    counter(1, {type: 'WHATEVER'})
).toEqual(1);

expect(
    counter(undefined, {})
).toEqual(0);

// we'll get here only if all tests pass
console.log('tests passed');