var App = React.createClass({
    getInitialState: function() {
        return {value: 4.5}
    },
    change: function(event) {
        this.setState({value: parseFloat(event.target.value)});
    },
    render: function() {
        return <input type="number" value={this.state.value} onChange={this.change}/>;
    }
});

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