[robhogan]

The worker's input Babel plugin writes function maps to metadata, so this removes the deprecated fallback to a functionMap returned by the transformer. A custom transformer that doesn't apply the plugins it's given loses function names in stack traces.

Changelog:

 - **[Breaking]**: Babel transformers can no longer return a `functionMap`. Apply the `plugins` passed by Metro and return the resulting Babel `metadata`

Test plan:
Existing tests. React Native's transformer doesn't return one, and @expo/metro-config has its own worker.

[github-actions]

Metro review: Looks good

Risk: low · Commit: 72d5611 · 8 commands run

This removes the deprecated functionMap fallback from the Babel transformer contract, so the worker reads function maps only from metadata.metro. A base/head comparison shows the only change is for a transformer that returns functionMap directly, while Metro's default transformer, @react-native/metro-babel-transformer 0.88.1 and the documented example all keep their maps. The description's ecosystem claims hold for the latest published packages, and the updated changelog and docs resolve my previous migration-note nit.

No findings.

Verified

• ✅ yarn jest --no-watchman packages/metro-transform-worker packages/metro-babel-transformer — 3 suites, 37 tests, 21 snapshots.
• ✅ yarn jest --no-watchman packages/metro/src/integration_tests/__tests__/buildGraph-test.js packages/metro/src/Server/__tests__/symbolicate-test.js — Covers function map consumers downstream of the worker.
• ✅ yarn jest --no-watchman packages/metro-transform-worker/src/__tests__/review-functionMap-test.js (head) — Legacy {ast, functionMap} gives null. Default, React Native (with and without hermesParser) and docs-example transformers give a real map.
• ✅ yarn jest --no-watchman packages/metro-transform-worker/src/__tests__/review-functionMap-test.js (base) — Same results as head except legacy, which returns its map. Transformers skipping plugins or dropping metadata were already null at base unless they returned functionMap.
• ✅ yarn typecheck
• ✅ yarn verify-api-snapshots — The API.md change matches the type change.
• ✅ yarn eslint packages/metro-transform-worker/src/index.js packages/metro-babel-transformer/src/index.js
• ✅ npx npm@11 pack @react-native/metro-babel-transformer@latest @expo/metro-config@latest react-native-svg-transformer@latest --pack-destination . — Run with the package_manager tool. RN 0.88.1 returns {ast, metadata}. @‍expo/metro-config 58.0.20 has its own worker and its transformer returns metadata. react-native-svg-transformer 1.5.3 forwards plugins.

Not run: No benchmark: the change removes one nullish fallback per Babel transform, with no plausible cost effect. Windows wasn't run: no path or platform behavior changes. The full Jest suite wasn't run, only the changed packages and the function map consumers listed. Ecosystem checks cover the latest versions of the three packages named, not older releases or other transformers. A first download command using a shell variable was denied by the permission policy, so npm view metadata wasn't collected; the package_manager tool fetched the packages instead.

_{Automated review by Claude. Advisory only: it never approves or blocks, and a maintainer makes the call.}