I'm generating sitemaps by having a root sitemap that checks through the fs to get the page structure for other sitemaps within the /sitemaps/ folder and creates a sitemapindex entry per sitemap. It does this within getServerSideProps each other's sitemap does the same to generate the urlset sitemap

All is working fine; however, I am struggling to get any tests working on it to prevent regression. Any help appreciated - some code snippets below to help paint the picture

const generateSitemap = (pagesXml: string) => {
    return `<?xml version="1.0" encoding="UTF-8"?>
        <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
            ${pagesXml}
        </urlset>`;
};

const generateSitemapIndex = (pagesXml: string) => {
    return `<?xml version="1.0" encoding="UTF-8"?>
    <sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
        ${pagesXml}
    </sitemapindex>`;
};

export const writeSitemap = (contents: string, type: 'urlset' | 'sitemapindex', context: GetServerSidePropsContext) => {
    const { res } = context;
    res.setHeader('Content-Type', 'text/xml');
    if (type === 'urlset') {
        res.write(generateSitemap(contents));
    } else if (type === 'sitemapindex') {
        res.write(generateSitemapIndex(contents));
    }
    res.end();
};

export const getServerSideProps: GetServerSideProps = async (context) => {
    const pages = [];
    const data = await fetchData();

    if (data) {
        Object.keys(data).forEach((item) => {
            Object.keys(data[item]).forEach((l10n) => {
                pages.push(baseUrl + routes.page(item.slug);
            });
        });

        const uniquePages = Array.from(new Set(pages));

        const pages = uniquePages
            .map(
                (url) =>
                    `<url>
                        <loc>${url}</loc>
                        <priority>0.7</priority>
                        <changefreq>daily</changefreq>
                    </url>`
            )
            .join('');

        writeSitemap(pages, 'urlset', context);
    }

    return {
        props: {},
    };
};