"domContainerNode is not defined." How do I fix this?

[echizenyayota]

Hi, everyone!
I want to show the button which displays "Button clicked!" on the screen using my index.js below.

import {AppProvider, Button} from '@shopify/polaris';
import React from 'react';
import ReactDOM from 'react-dom';

const app = (
<Button onClick={() => alert('Button clicked!')}>Example button</Button>
);

ReactDOM.render(app, domContainerNode);

export default app;

But then, the screen shows me "domContainerNode is not defined."

How do I fix the above to display the button?
In addition, I have already published the other files to display the button on GitHub.