redux-subscribe.js

//App.js
import React, { Component } from 'react';

import {incrementCount} from '../actions';

export default class App extends React.Component {
  constructor(props){
    super(props);

    this.state = {
      count: props.store.getState()
    };

    this.handleClick = this.handleClick.bind(this);
    this.handleChange = this.handleChange.bind(this);
  }

  componentDidMount(){
    this.props.store.subscribe(this.handleChange);
  }
  
  handleChange(){
    this.setState({
      count: this.props.store.getState()
    });
  }

  handleClick(){
    this.props.store.dispatch(incrementCount());
  }

  render(){
    return (
      <div>
        <p>Count: {this.state.count}</p>
        <button onClick={this.handleClick}>Increment</button>
      </div>
    );
  }
}

//index.js
import React, { Component } from 'react';
import ReactDOM from 'react-dom';
import App from './components/App';

import { createStore } from 'redux';
import reducer from './reducers';

let store = createStore(reducer);

ReactDOM.render(<App store={store}/>, document.getElementById('app'));