App.js

import React, { Component } from 'react';
import ReactDOM from 'react-dom';

/**
 * Creates the component that will be tested with 
 * Jest and Enzyme
 */
class MessageDisplay extends Component {
  constructor(props) {
    super(props);
    this.handleClick = this.handleClick.bind(this);

    this.state = {
      index: 0,
      texts: ['Jest', 'I ❤ React', 'Enzyme'],
    };
  }

  handleClick() {
    const index = this.state.index;
    const updatedIndex = (index < 2) ? (index + 1) : 0;

    this.setState({
      index: updatedIndex,
    });
  }

  render() {
    const { index, texts } = this.state;

    return (
      <div>
        <button onClick={this.handleClick}>
          Click to change the text
        </button>
        <span>
          { texts[index] }
        </span>
      </div>
    );
  }
}

/**
 * Renders the component on the DOM
 */
ReactDOM.render(<MessageDisplay />, document.getElementById('root'));