This guy made a Javascript library running on WebGPU (an upcoming web GPU wrapper) that uses the coding style popularized by React.

React was designed to make it easy(-ish) to dynamically update web pages. When data or properties are changed in the JS code, React updates the DOM (HTML that tells the browser what to draw on screen). That's reactive.

Declarative is how it's organized. To change what shows up on screen, there has to be some kind of update loop that checks for/reacts to (pull/push) new data. React hides all that, the idea is you declare what you want drawn once, and it's updated for you as the data changes.

React stays efficient by only updating what needs to be updated in the DOM. This gets a similar result by memoizing (caching function return values) (for what data??).

Editorializing from my experience with web frameworks, that style feels magical when it works, and makes for flashy demos (look what I can do with 10 lines of code!), but it gets there by hiding a lot of complexity inside an opinionated framework. It's way easier to get started with, but you have to be disciplined and know how it works under the hood to stay on the happy path with larger projects.