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.
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?