How to get absolute URLs in a React Static sitemap

When I first generated used react-static-plugin-sitemap, it outputted a sitemap (at the URL /sitemap.xml) with URLs that are relative, meaning that they only start with /.

Sitemaps aren't supposed to have relative URLs, they're supposed to have full URLs (which start with http:// or https:// and include the domain).

This problem arises because React Static doesn't know which domain your site is going to be published at. You can fix this by adding a property to the object exported by static.config.js:

// static.config.js

export default {
	siteRoot: 'https://example.com'
}

Now, the URLs in sitemap.xml will have the full domain in them.