rxreact.js

var Game = React.createClass({
  render: function() {
    var x = this.props.x,
        y = this.props.y;

    return (
      <svg width={Math.max(x + 10, 200)} height={height: Math.max(y + 10, 200)}>
        <circle cx={50} cy={50} r={2} fill="#000" />
        <circle cx={x} cy={y} r={2} fill="#000" />
        <line x1={50} y1={50} x2={x} y2={y} stroke="#000" />
      </svg>
    );
  }
});

var moves = Rx.Observable.fromEvent(document, 'mousemove'),
    downs = Rx.Observable.fromEvent(document, 'mousedown'),
      ups = Rx.Observable.fromEvent(document, 'mouseup');

downs.flatMap(function() {
  return moves.takeUntil(ups);
}).merge(downs).map(function(event) {
  return {x: event.x - 10, y: event.y - 10};
}).subscribe(function(props) {
  React.renderComponent(Game(props), document.body);
});