How to implement a sitemap.xml file for a single page app using Firebase?

Question

I was reading Google's guidelines about SEO and I found this.

Help Google find your content

The first step to getting your site on Google is to be sure that Google can find it. The best way to do that is to submit a sitemap. A sitemap is a file on your site that tells search engines about new or changed pages on your site. Learn more about how to build and submit a sitemap.

Obs.: My web app is an ecommerce/blog in which I have a shop that I have products to sell and I have a blogging section where I create and post content about those products.

So, each product has a product page, and each blog post has a blogPost page.

Then I went looking for some examples of Sitemaps from websites like mine that have good SEO ranking.

And I've found this good example:

robots.txt

User-Agent: *
Disallow: ... // SOME ROUTES

Sitemap: https://www.website.com/sitemap.xml

I.E: Apparently the crawler robot finds the Sitemap location from the robots.txt file.

And I've also found out that they keep separate sitemap files for blogPost and product pages.

sitemap.xml

<sitemapindex xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/siteindex.xsd">
  <sitemap>
    <loc>https://www.website.com/blogPosts-sitemap.xml</loc> // FOR POSTS
    <lastmod>2019-09-10T05:00:14+00:00</lastmod>
  </sitemap>
  <sitemap>
    <loc>https://www.website.com/products-sitemap.xml</loc>  // FOR PRODUCTS
    <lastmod>2019-09-10T05:00:14+00:00</lastmod>
  </sitemap>
</sitemapindex>

blogPosts-sitemap.xml

// HUGE LIST WITH AN <url> FOR EACH BLOGPOST URL

<url>
  <loc>
    https://www.website.com/blog/some-blog-post-slug
  </loc>
  <lastmod>2019-09-03T18:11:56.873+00:00</lastmod>
  <changefreq>weekly</changefreq>
  <priority>0.8</priority>
</url>

products-sitemap.xml

// HUGE LIST WITH AN <url> FOR EACH PRODUCT URL

<url>
  <loc>
    https://www.website.com/gp/some-product-slug
  </loc>
  <lastmod>2019-09-08T07:00:16+00:00</lastmod>
  <changefreq>yearly</changefreq>
  <priority>0.3</priority>
</url>

QUESTION

How can I keep updated Sitemap files like that if my web app is a Single Page App with client site routing?

Since I'm using Firebase as my hosting, what I've thought about doing is:

OPTION #1 - Keep sitemap.xml in Firebase Hosting

From this question Upload single file to firebase hosting via CLI or other without deleting existing ones?

Frank van Puffelen says:

Update (December 2018): Firebase Hosting now has a REST API. While this still doesn't officially allow you to deploy a single file, you can use it creatively to get what you want. See my Gist here: https://gist.github.com/puf/e00c34dd82b35c56e91adbc3a9b1c412

I could use his Gist to update the sitemap.xml file and run this script once a day, or whenever I want. This would work for my current project, but it would not work for a project with a higher change frequency of dynamic pages, like a news portal or market place, for example.

OPTION #2 - Keep sitemap.xml in Firebase Storage

Keep the sitemap files in my Storage bucket and update it as frequently as I need via a admin script or a cloud scheduled function.

Set a rewrite in my firebase.json and specify a function to respond and serve the sitemap files from the bucket, when requested.

firebase.json

"hosting": {
 // ...

 // Add the "rewrites" attribute within "hosting"
 "rewrites": [ {
   "source": "/sitemap.xml",
   "function": "serveSitemapFromStorageBucket"
 } ]
}

FINAL QUESTION

I'm leaning towards OPTION #2, I want to know if it will work for this specific purpose or if I'm missing something out.

Answer 1

I ended up creating a cloud function to build the sitemap file on-demand.

firebase.json

"rewrites": [
  {
    "source": "/sitemap.xml",
    "function": "buildSitemap"
  },
]

buildSitemap.js (this is a cloud function)

import * as admin from 'firebase-admin';

async function buildSitemap(req,res)  {

  // Use firebase-admin to gather necessary data
  // Build the sitemap file string
  // and send it back

  res.set('Content-Type', 'text/xml');
  res.status(200).send(SITEMAP_STRING);
  return;

}

export default buildSitemap;