<div id="app"></app>

class Todo extends React.Component {

    componentDidMount() {
        setInterval(() => {
            this.setState(() => {
                return { unseen: "does not display" }
            });
        }, 1000);
    }

    render() {
        console.log('render called');
        return (<div>...</div>);
    }
}

React.render(<Todo />, document.getElementById('app'));