I think what we need is a very thin layer, or perhaps just a binding, between JavaScript, and native UI API calls. Then you could have a JavaScript non-web app with native look and feel
I'm not sure if I'm reading this right but either duktape or WebView might do what you want.
I'm thinking of a world with Javascript, but without HTML or a web server.
You're describing the Mozilla platform's old XUL strategy, or the modern Gnome desktop. JS has permeated GTK for the last 10 years. It was a huge part of Gnome 3. For a brief moment, pushing people to start with JS for their next GTK app was an explicit strategy for the Gnome project. There was so much backlash, though, that they backed away from that position a month or two later. Had they had the courage to stand their ground, the rise of Electron might never have happened. A world with a better GTK as the go-to cross-platform UI toolkit would have been a lot better than where we ended up instead.
Or Qt’s QML.
You can make a hello-world app with JS and GTK by creating a file hello.js, typing some code in, and then running it. AFAIK, although Qt has some support for JS integration, the story is not the same. You still need to develop a traditional Qt app and then start peppering in JS where it suits you. Am I wrong?
I did a little proof-of-concept a long time ago using Mozilla Rhino + SWT apis and managed to get a small window on the screen with a few buttons with only javascript, but I couldn't figure out the api to make a simple http call...