Embedable js app that talks to Rails

Hello, I wonder if anybody has some thoughts on how to do this.

I need to rewrite an iFrame app that runs embeded in various websites so that it runs as a js app in a div rather than an iFrame. This is because of the many problems with cookies and ITP.

The JS app will talk to our rails app to get data from an api and/or html.

What frameworks / methodoigies do people recommend?