Since you are already a seasoned developer you can cut through a lot of the blog posts and courses that are aimed at junior developers. The two main resources I find myself reaching for are the Mozilla JS docs, and the React docs. React team is currently working on a new version of the docs, googling “React docs beta” should get you what you want.

As for things to learn, async/await and promises are probably the most complicated, but Mozilla will no doubt have documentation covering all of that.

For React, prefer small contained functional components, avoid classes and inheritance, and learn what hooks are and how they work.

For Typescript, avoid using any or unknown everywhere, avoid the urge to make very complex types, and generally stick to interfaces with minimal inheritance and you’ll be fine.

As you are jumping to an existing codebase follow the conventions of that codebase. Avoid the classic blunder of jumping into a new codebase and making massive changes.

Good luck!