App.js

import React, { Component } from "react";
class App extends Component {
  render() {
    return (
      <div className="App">
        <header className="App-header">Teste Redux</header>
      </div>
    );
  }
}

export default App;

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(<App />, document.getElementById('root'));