Inline the last duplicate Platform.select key - #58249
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Let's discuss this at react/metro#1889. |
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@vzaidman has imported this pull request. If you are a Meta employee, you can view this in D118499490. |
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Summary:
Metro statically replaces `Platform.select({...})` for the target platform. Its property scan currently stops at the first matching key, while JavaScript object-literal evaluation keeps the last duplicate definition. For example, `Platform.select({ios: 1, ios: 2})` runs as `2` but Metro compiles it to `1`.
Scan the already-validated static properties from the end and return the first reverse match. This preserves O(n) behavior, avoids an unnecessary nullable temporary, and keeps compiled output consistent with runtime object semantics. The React Native Babel preset contains a parallel transform; companion [React Native PR #58249](react/react-native#58249) applies the same rule there.
Changelog:
```
- **[Fix]**: Inline the last duplicate key from static `Platform.select` object literals
```
Pull Request resolved: #1889
Test Plan:
- Added a focused `metro-transform-plugins` regression; it fails on pristine main with `1` and passes with `2` after the fix.
- All package suites pass: 6/6 suites, 160/160 tests, 41 snapshots.
- Full Flow check reports no errors.
- Full monorepo build passes for all 17 packages.
- Targeted ESLint, Prettier, and `git diff --check` pass.
No behavior changes for object literals without duplicate static keys.
Reviewed By: GijsWeterings
Differential Revision: D118499357
Pulled By: vzaidman
fbshipit-source-id: 1036c78826c4e7d12a94baa8b8eeb1a04f9f8f60
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Summary:
The React Native Babel preset statically replaces
Platform.select({...})for the target platform. Its property scan currently stops at the first matching key, while JavaScript object-literal evaluation keeps the last duplicate definition. For example,Platform.select({ios: 1, ios: 2})runs as2but the preset compiles it to1.Scan the already-validated static properties from the end so compiled output matches runtime semantics while preserving O(n), allocation-free lookup. Metro has a parallel transform that can run first; companion Metro PR #1889 applies the same correction so output remains transform-order independent.
Changelog:
[GENERAL] [FIXED] - Inline the last duplicate key from static Platform.select object literals.
Test Plan:
const value=1, while the fix emitsconst value=2.git diff --checkpass.No behavior changes for object literals without duplicate static keys; no UI change.