callbackRefClass.js

class App extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
      value: 'highlight and copy me'
    }
    this.handleClick = this.handleClick.bind(this);
  }
 
  handleClick() {
    this.textInput.select();
    document.execCommand('copy');
  }
  
  render() {
    return (
      <div>
        <input type="text" ref={el => this.textInput = el} readOnly value={this.state.value}/>
        <button onClick={this.handleClick}>Copy</button>
      </div>
    );
  }
}