Applications built with just React usually have a single root DOM node. If you are integrating React into an existing app, you may have as many isolated root DOM nodes as you like. To render a React element into a root DOM node, pass both to ReactDOM.render():

    const element = <h1>Hello, world</h1>;
    ReactDOM.render(element, document.getElementById('root'));
        

React elements are immutable. Once you create an element, you can’t change its children or attributes. The only way to update the UI is to create a new element, and pass it to ReactDOM.render().

    function tick() {
        const element = (
            <div> <h1>Hello, world!</h1> <h2>It is {new Date().toLocaleTimeString()}.</h2> </div>
        );
        ReactDOM.render(element, document.getElementById('root'));
    }

    setInterval(tick, 1000);