If you'd run this code in your browser console:

1import('https://cdn.jsdelivr.net/npm/@thumbmarkjs/thumbmarkjs@rc/dist/thumbmark.umd.js')
2.then(() => {
3  const tm = new ThumbmarkJS.Thumbmark();
4  tm.get().then((res) => {
5      console.log(res)
6  })
7})

You should see something like this:

1{}

So what is this?

ThumbmarkJS is a client-side browser fingerprinting library that generates a unique identifier based on various browser and device characteristics. This identifier, called a thumbmark, is designed to be stable across sessions and resistant to simple anti-fingerprinting techniques. It looks like this: loading....

The main purpose of ThumbmarkJS is to provide a reliable way to identify users without relying on cookies or other tracking methods that can be easily blocked or deleted. Thumbmark also provides some basic classification of the client, such as whether it is a Tor exit node, VPN, or datacenter IP.

Head over to the ThumbmarkJS GitHub repository for the actual documentation.