class App extends React.Component {
    state = {
    value: ''
  }
  handleSubmit = e => {
    e.preventDefault();
    this.setState({ value: this.textInput.value})
  };

  render() {
    return (
      <div>
        <h1>React Ref - Callback Ref</h1>
        <h3>Value: {this.state.value}</h3>
        <form onSubmit={this.handleSubmit}>
          <input type="text" ref={e => this.textInput = e} />
          <button>Submit</button>
        </form>
      </div>
    );
  }
}

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