class FileInput extends React.Component {
  // highlight-range{4}
  constructor(props) {
    super(props);
    this.handleSubmit = this.handleSubmit.bind(this);
    this.fileInput = React.createRef();
  }
  // highlight-range{4}
  handleSubmit(event) {
    event.preventDefault();
    alert(
      `Selected file - ${this.fileInput.current.files[0].name}`
    );
  }

  render() {
    return (
      <form onSubmit={this.handleSubmit}>
        <label>
          Upload file:
          {/* highlight-range{1-3} */}
          <input
            type="file"
            ref={this.fileInput}
          />
        </label>
        <br />
        <button type="submit">Submit</button>
      </form>
    );
  }
}

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