One special thing about
ReactDOM.render is that it only updates DOM elements that have changed.
That means that if you render the exact same thing twice in a row, the second render will do nothing:
var hello = <h1>Hello world</h1>;
This is significant! Only updating the necessary DOM elements is a large part of what makes React so successful.
React accomplishes this thanks to something called the virtual DOM. Before moving on to the end of the lesson, read this article about the Virtual DOM.