function bits(n){

  var res = [];

  while(n){

    res.push(n & 1);

    n >>= 1;

  }

  return res;

}

​

class App extends React.Component {

  constructor(){

    super();

    this.state = {buttons: 0};

  }

  render(){

    const preventDefault = (e) => { e.preventDefault() }

    const handleChange = (e) => {

      preventDefault(e);

      this.setState({buttons: e.buttons});

    };

    var names = [

      'Left', 'Right', 'Middle', 'Back', 'Forward'

    ];

    // buttons is a bitmask

    var pad = "00000";

    var buttons = bits(this.state.buttons).concat([0, 0, 0, 0, 0]).slice(0, names.length);

    var active = buttons.map((x, i) => {

      return <div>{names[i]}: {x === 1 ? 'Yes' : 'No'}</div>;

    });

    return (

      <section 

        onContextMenu={preventDefault}

        onMouseDown={handleChange}

        tabIndex={-1}>

        <div>

          <h4>Buttons: {this.state.buttons} ({buttons.join('')})</h4>

          {active}

        </div>

      </section>

    );

  }

}

​

React.render(<App />, document.body);