https://htmx.org - uses HTML attributes and HTML-over-the-wire for AJAX/Web Socket/SSE interactions
https://hyperscript.org - an experimental front end programming language derived from HyperTalk that is event-oriented and that removes the distinction between synchronous and asynchronous code.
Both tools are HTML-oriented and are designed to be embedded directly in your HTML, rather than along side it. I am trying to popularize the term "Locality of Behaviour" for this idea:
https://htmx.org/essays/locality-of-behaviour/
There are a few tools that are moving people this direction these days, notably AlpineJS and Tailwinds.