counter.js

var Counter = React.createClass({
  getInitialState: function() {
    window.aCounter = this;
    return {
      count: 0
    };
  },
  
  add: function() {
    this.setState({count: this.state.count+1});
  },

  subtract: function() {
    this.setState({count: this.state.count-1});
  },

  render: function() {
    return (
      React.DOM.div({
        children: [
          React.DOM.label({children: this.state.count}),
          React.DOM.button({onClick: this.add, children: "+"}),
          React.DOM.button({onClick: this.subtract, children: "-"})
        ]
      })
    );
  }
});

React.renderComponent(
  React.DOM.div({
    children: [
      React.DOM.h1({children: "A Counting Widget!"}),
      Counter()
    ]
  }),
  document.getElementById('counters')
);

var s = new Date();
for(var i = 0; i < 10000; i++) {
  aCounter.add();
}
console.log((new Date())-s);