[0z5a]

Summary

flattenBlockList turns a blockList array into a single RegExp by joining the sources of its patterns with |. For an empty array the result is new RegExp(''), which matches every string. metro-file-map then adds its VCS directory pattern ((?:)|[/\\]\.(git|hg)[/\\]), and that still matches every path. So resolver: {blockList: []} gives an empty file map, and the first build fails with Failed to get the SHA-1 for: <projectRoot>/index.js.

The docs say setting blockList replaces the default, so an empty list should mean that nothing is blocked. Server-test.js already uses blockList: [] that way; it only passes because it mocks DependencyGraph. The existing regexes[0]?.flags ?? '' shows that an empty array was expected here.

This returns a pattern that never matches (/.^/, the same one resolver-test.js uses) for an empty array. Non-empty arrays and single RegExps are unchanged.

Changelog: [Fix] resolver.blockList: [] no longer hides every file from Metro

Test plan

I added a blockList config param block to packages/metro/src/DeltaBundler/__tests__/resolver-test.js. It builds a real DependencyGraph on the in-memory fs (linux and win32):

• a blockList array hides a matching file and leaves the others resolvable (passes before and after; it's a control)
• an empty blockList array hides nothing (fails before this change, passes after)

$ yarn jest packages/metro/src/DeltaBundler/__tests__/resolver-test.js -t 'blockList config param'

Before:

      ✓ hides files matching a pattern in a blockList array
      ✕ does not hide any file with an empty blockList array
      ✓ hides files matching a pattern in a blockList array
      ✕ does not hide any file with an empty blockList array
    Unable to resolve module ./a from C:\root\index.js:
    None of these files exist: ...
Tests:       2 failed, 262 skipped, 2 passed, 266 total

After:

      ✓ hides files matching a pattern in a blockList array
      ✓ does not hide any file with an empty blockList array
      ✓ hides files matching a pattern in a blockList array
      ✓ does not hide any file with an empty blockList array
Tests:       262 skipped, 4 passed, 266 total

Other checks:

$ yarn jest packages/metro/src/DeltaBundler/__tests__/resolver-test.js packages/metro/src/node-haste packages/metro/src/Server/__tests__/Server-test.js packages/metro-config packages/metro-file-map
Test Suites: 44 passed, 44 total
Tests:       13 skipped, 1278 passed, 1291 total
$ yarn flow check
No errors!
$ yarn eslint <changed files>   # clean
$ yarn prettier --check <changed files>   # clean

I also checked it end to end with Metro.runBuild on a two-file project. With blockList: [] the build failed with Failed to get the SHA-1 for: .../index.js before this change. After it, the bundle builds and runs, and the default, a one-pattern array and a single RegExp give the same results as before.

This change was drafted with Claude Code (claude-opus-5-5); the test results above are from the runs described.

[github-actions]

Metro review: Looks good

Risk: low · Commit: 1f9e4bf · 4 commands run, in a restricted sandbox after an automated safety assessment

With resolver.blockList: [], flattenBlockList used to build new RegExp(''), which matches every path, so the file map came out empty. This PR returns a never-matching /.^/ for an empty array instead. The fix is correct and minimal: non-empty arrays and single RegExps take the same code path as before. metro-file-map still appends its VCS-directory pattern to /.^/ as intended. The new test fails on the base and passes on the head.

No findings.

Verified

• ✅ yarn jest --no-watchman packages/metro/src/DeltaBundler/__tests__/resolver-test.js packages/metro/src/node-haste (head) — 5 suites, 276 tests passed
• ✅ yarn jest --no-watchman packages/metro/src/DeltaBundler/__tests__/resolver-test.js -t 'blockList config param' (base code with the PR's test file copied in) (failed as expected) — Fails on base: the empty-blockList test fails on both platforms and the array control passes, as the description says
• ✅ yarn eslint packages/metro/src/node-haste/DependencyGraph/createFileMap.js packages/metro/src/DeltaBundler/__tests__/resolver-test.js
• ✅ node -e (builds metro-file-map's combined ignorePattern from /.^/ plus VCS_DIRECTORIES and tests it on paths) — A regular file is not ignored; /.git/ and .hg\ paths are still ignored

Not run: No benchmark: the change only runs once at file map construction, and only for an empty array, so it can't plausibly affect cost. I didn't run yarn typecheck. The function's inferred return type is still RegExp, and the description reports a clean Flow run. The file map cache key changes only for configs with an empty blockList, which invalidates their previously broken, empty cache. That's desirable. No Windows run: separators don't matter to this logic, and the new test already runs under the win32 mock fs.

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