Let’s examine the code that you just wrote in the last exercise.

ReactDOM.render(<h1>Render me!</h1>, document.getElementById('app'));

You can see something called ReactDOM. What’s that?

ReactDOM is the name of a JavaScript library. This library contains several React-specific methods, all of which deal with the DOM in some way or another.

We’ll talk more later about how ReactDOM got into your file. For now, just understand that it’s yours to use.

Move slightly to the right, and you can see one of ReactDOM‘s methods: ReactDOM.render().

ReactDOM.render() is the most common way to render JSX. It takes a JSX expression, creates a corresponding tree of DOM nodes, and adds that tree to the DOM. That is the way to make a JSX expression appear onscreen.

Move to the right a little more, and you come to this expression:

<h1>Hello world</h1>

This is the first argument being passed to ReactDOM.render(). ReactDOM.render()‘s first argument should be a JSX expression, and it will be rendered to the screen.

We’ll discuss the second argument in the next exercise!