5 FAQs about React Compiler

A short and quick look at some of the commonly asked questions about React Compiler

Question 1: Do I need to remove my useMemo and useCallback now?

No. It is safe to leave them in. Compiler does its own low-level memoization, and understands and checks manual memoization for correctness.

Question 2: Is there a difference between the memoization of useMemo, useCallback and React Compiler?

Yes. Compiler uses memoization techniques different from the memoizations of `useMemo` and `useCallback`. Specifically, the Compiler uses an internal React hook `_c` formerly called `useMemoCache`.

Question 3: Does the Compiler need React 19?

No. The compiler does not ship with React 19 . You can use it with other React versions by installing react-compiler-runtime.

Question 4: Are there ways to still manually optimize some files?

Yes. Use “use no memo“ in files you want the Compiler to skip.

Question 5: How do I know if the code is compiled by the compiler?

1. If you are using React Devtools (v5.0+), you will notice a “Memo ✨” badge next to components that have been optimized by the compiler.

2. If you paste your code in the React Compiler Playground , you will notice the functions get transformed by the `_c` hook, which denotes that compiler has optimized the code. Here’s an example of what that looks like -

  const $ = _c(9);
  const { friends } = t0;
  const onlineCount = useFriendOnlineCount();
  if (friends.length === 0) {
    let t1;
    if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
      t1 = <NoFriends />;
      $[0] = t1;
    } else {
      t1 = $[0];
    }
    return t1;
  }

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Got more questions about the React Compiler? Let me know in the comments, and I’ll cover them in the next newsletter!