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[Compiler Bug]: false negative, compiler skips memoizing a variable that should be memoized | GoodFirstPicks

[Compiler Bug]: false negative, compiler skips memoizing a variable that should be memoized

facebook/react 0 comments 1mo ago
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highopenScope: somewhat clearSkill match: maybeReactJavaScriptTypeScript

Why this is a good first issue

Compiler optimization issue requiring deep understanding of React internals.

AI Summary

The React Compiler fails to memoize a variable (`processedData`) that should be memoized, leading to unnecessary recalculations when `onClick` changes. This requires investigating and fixing the compiler's optimization logic. The issue involves understanding React's compiler internals and may have broader implications.

Issue Description

What kind of issue is this?

  • React Compiler core (the JS output is incorrect, or your app works incorrectly after optimization)
  • babel-plugin-react-compiler (build issue installing or using the Babel plugin)
  • eslint-plugin-react-hooks (build issue installing or using the eslint plugin)
  • react-compiler-healthcheck (build issue installing or using the healthcheck script)

Link to repro

https://playground.react.dev/#N4Igzg9grgTgxgUxALhAMygOzgFwJYSYAEAogB4AOCmYeAbggMIQC2Fh1OAFMEQCYBDHAIA0RQowA2eOAGsiAXwCURYAB1iROITA4iFGBERgwCPgBEhAogF4iCStVoMACoeO1MAcy6DhSgG4NDSItHT0ACwFMPkkmaTlbIi48HAQWFRsAPlUQ0PFMKRlZFLSWADo8PkC8hSDMPJgEHFhiLjzQgB4+eiyO-OADIwQTM0thcpYBClL02xz2zXz8zoBJMqJZBABPG2BU9Mq+BQKiuT2uTJyomLizkoOMk4B6PqX8pSUFfs7nnro3qEapgFCARCBtJg0HgvCgQHg2BAYHocNsqKoiC5JFAvHhMAB5Cj4HSKIhoQwsIgAcgARgIaQhJABaCjY3GYJlNAS4JnaNh4OIwP54XRU+oaHh5Z7PPkUAVCAiYACyED4CGQRDUIAEkkkWo0JzACrA0JGmLZeMJxJogTB4AiEAA7qtMGkYJgdWAUGhPQgFEA

Repro steps

Open the example from React Compiler introduction , which is linked by See this example in the React Compiler Playground

The example Component itself expects props with two properties: function ExpensiveComponent({ data, onClick }) This example from React Compiler docs doesn't memoize the first line in its body const processedData = expensiveProcessing(data);

If you look closely at the compiled output, the compiled version would recalculate that line each time when onClick is changed, even though data is the same.

If we manually add useMemo as it was in the original example fro

GitHub Labels

Type: BugStatus: Unconfirmed

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Risk Flags

  • compiler internals
  • potential cross-cutting changes
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Details

Points30 pts
Difficultyhigh
Scopesomewhat clear
Skill Matchmaybe
Test Focusedno