Introduction to React 19: Compiler & Build-Time Optimizations

Introduction to React 19: Compiler & Build-Time Optimizations

One of the most ambitious projects in the React ecosystem is the React Compiler, code-named “React Forget”, which aims to automatically optimize React code at build time. While not fully GA in React 19, there have been significant updates:

  • React Compiler (Forget) Beta: The React team has been working on a new compiler that can automatically insert memoization to “forget” about re-rendering unchanged parts of your UI. In practical terms, this could eliminate the need for manual React.memo, useMemo, and useCallback in many cases (React 19 is on its way — What’s changing? | by Hvardhan | Medium). As of React Conf 2024, the compiler was announced in experimental form, and with React 19 the project has matured into a beta release available for early adopters (React Compiler Beta Release – React) (React Compiler Beta Release – React). To try it, you install babel-plugin-react-compiler@beta and an ESLint plugin (React Compiler Beta Release – React) (React Compiler Beta Release – React). The compiler runs at build time and transforms your code to add optimizations that make React skip re-renders when possible. It’s essentially an automation of what we’d normally do with useMemo/useCallback.

  • Backwards Compatibility via Runtime: Initially, the compiler’s output required React 19’s new runtime APIs, but React 19 introduced a react-compiler-runtime package to backport support to React 17 and 18 apps (React Compiler Beta Release – React) (React Compiler Beta Release – React). This means you don’t have to be on React 19 to experiment with the compiler – you can specify a target React version for the compiled output. The runtime polyfill will provide any missing functions in older React so that the compiled code runs correctly (React Compiler Beta Release – React) (React Compiler Beta Release – React). For library authors, the recommendation is to compile your libraries with the React Compiler and ship the optimized code, so apps using your library benefit even if they aren’t themselves using the compiler (React Compiler Beta Release – React) (React Compiler Beta Release – React).

  • Automatic Memoization & “Rules of React”: A big part of making the compiler effective is ensuring your code follows certain patterns. The React team published the “Rules of React” (similar to Rules of Hooks) to help identify code that might confuse the compiler. To assist, an ESLint plugin for React Compiler is available and the team strongly recommends using the linter today (React Compiler Beta Release – React). This linter flags things that would prevent the compiler from safely optimizing (such as side-effects in render). By adopting it, you can incrementally refactor your code to be “compiler-ready.” The upside: down the road, turning on the compiler could yield performance gains with minimal code changes. In fact, the React team has indicated a future where we “say goodbye to useMemo and useCallback” because the compiler does it for us (React 19 is on its way — What’s changing? | by Hvardhan | Medium) (React 19 is on its way — What’s changing? | by Hvardhan | Medium). That future isn’t fully here yet, but React 19 lays important groundwork.

  • Real-World Usage & Results: Even in beta, the React Compiler has seen extensive real-world testing at Meta (Facebook). It has been rolled out internally to apps like Instagram, Facebook Web, and Threads, comprising tens of thousands of components (React Compiler Beta Release – React) (React Compiler Beta Release – React). The reports are encouraging – performance improvements in the order of a few percent across already highly optimized codebases (React Compiler Beta Release – React) (React Compiler Beta Release – React). That may sound small, but at the scale of these apps (and given how optimized they already were), those are significant wins. Additionally, Meta found that developer productivity improved by removing the mental overhead of manually optimizing components (React Compiler Beta Release – React). These results suggest that when the compiler becomes stable for everyone, even moderate-sized apps could benefit from improved rendering performance with much less effort from developers.

  • Future Roadmap: The React team has opened up the React Compiler Working Group to public participation (React Compiler Beta Release – React) (React Compiler Beta Release – React). This means they are seeking community feedback and preparing ecosystem tooling (like Babel, bundlers, etc.) for eventual stable adoption of the compiler. While React 19 does not ship the compiler by default, it represents a turning point where the compiler is no longer a research experiment but a concrete tool moving towards production readiness. Developers keen on the cutting edge can try it out, run the linter, and start aligning their code with the patterns it expects, knowing that a future React (possibly React 20) might make these optimizations mainstream.

Conclusion

In summary, the React Compiler is on the horizon to make React apps faster automatically. React 19 provides optional tools to test it out and ensures that whenever you do upgrade to React 19+, your app is compiler-compatible. As one early article put it: “Enter ‘react-forget,’ the React compiler that’s set to change the game by handling memoization automatically… no more manual useMemo or useCallback” (React 19 is on its way — What’s changing? | by Hvardhan | Medium) (React 19 is on its way — What’s changing? | by Hvardhan | Medium). It’s not fully plug-and-play yet for everyone, but it’s one of the most exciting developments in the React ecosystem.

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