ReactJS 19: The Significant Parts

1. Concurrent Mode Enhancements:

   The biggest improvements in ReactJS 19 have been made with Concurrent Mode, which not only keeps the UI smooth and responsive while your application updates itself but also ensures a seamless interface especially at complex transitions such as animations.

2. Improved Server Components:

   Following the lead of Python, in ReactJS 19 Server Components were further advanced with improved support and tooling to architect reliable and performant server-rendered components.

3. Automatic Batching:

   Changes to state/props are now automatically batched across asynchronicity events, reducing re-renders and generally speaking perform a lot better with much less need on the developers side.

4. New JSX Transform:

   The previous version the opened up JSX Transform also got several updates bringing better TypeScript and modern JavaScript tooling integration, along with bundle size reduction and build improvements.