A (ultra) simple Counter app using Redux and React.

counter_reduxy_react.js

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

// the reducer is a pure function which is the responsible for changing
// the state of the application (without mutating the parameters)
const counter = (state = 0, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state - 1;
    default:
      return state;
  }
};

const store = createStore(counter);

// Creates a React component as a function.
const Counter = ({value, onIncrement, onDecrement}) => (
  <div>
    <h1>{value}</h1>
    <button onClick={onIncrement}>+</button>
    <button onClick={onDecrement}>-</button>
  </div>
);

const render = () => {
  ReactDOM.render(
    <Counter
      value={store.getState()}
      onIncrement={ () => store.dispatch({ type: 'INCREMENT'} )}
      onDecrement={ () => store.dispatch({ type: 'DECREMENT'} )}
    />, document.getElementById('app')
  );
};

store.subscribe(render);
render();

// It's worth noting that the size of my bundle file increased from ~1kB when
// implementing the store myself to ~2kB when using Redux's store and then
// finally to ~700kB when adding React + ReactDOM packages.