Resolver: Resolve the package of a directory once for all extension candidates by robhogan · Pull Request #1945 · react/metro

robhogan · 2026-09-18T15:39:03Z

An internal resolver refactor that improves resolution performance by ~30% and unblocks removing lookup caching, which unblocks incremental resolution and on-demand FS.

Context

When resolving a file, the resolver checks for package.json redirections once per candidate extension, asking for the package scope of each candidate path in turn:

metro/packages/metro-resolver/src/resolve.js

Lines 827 to 834 in 6dbe979

    
           function resolveSourceFileForExt( 
        
             context: SourceFileContext, 
        
             extension: string, 
        
           ): SourceFileResolution { 
        
             const filePath = `${context.filePathPrefix}${extension}`; 
        
             const redirectedPath = 
        
               // Any redirections for the bare path have already happened 
        
               extension !== '' ? redirectModulePath(context, filePath) : filePath;

All extension candidates in a directory have the same package scope, but we look each one up separately. And even though PackageCache memoises lookups it does so by module path and only caches hits, and since most candidates are files that don't exist, each one misses the first time.

This is the first of three PRs cutting the cost of closest-package lookups, before observations are collected per resolution in #1949.

This change

This looks up the package scope of the files in the directory once, through the directory's package.json, and shares it across the candidates. Resolver output is identical.

It cuts package scope lookups 3.6x (64k -> 18k per pass on our benchmark app) and takes 33% off first-pass resolution time (130ms -> 87ms). Warm passes are within a few percent, where PackageCache was already hitting.

Also documents what getPackageForModule returns for symlinked and missing paths.

Benchmark

AI-driven, 7 interleaved rounds, resolution measured in isolation.

metric	main	this PR
`getPackageForModule` calls per pass	64k	18k (3.6x fewer)
first pass after startup (ms)	130.4	87.3 (-33.1%)
warm pass (ms)	57.3	54.7 (-4.5%)
after a source change (ms)	61.0	59.4 (-2.6%)
after a `package.json` change (ms)	65.9	60.7 (-7.8%)

Changelog: Internal

Test plan

New unit test pins the number of getPackageForModule calls for the common specifier shapes.

Benchmark app (AI-driven harness, used throughout this stack): a synthetic RN 0.87 app with 23 common dependencies and ~1,000 generated first-party modules, 60% of them in a workspace package outside projectRoot with its own node_modules. 2.8k modules in the iOS dev bundle, 34k files in the file map. Timings replay the resolutions of a real build against a fresh DependencyGraph and isolate resolution. A full build of this app takes ~4s with a warm transform cache, so resolution is ~2% of it.

…one map lookup Stacked on #1945. ## Summary `hierarchicalLookup` walks up from a start path asking whether each ancestor contains a given subpath - it's how `DependencyGraph` finds the closest `package.json`. For every candidate ancestor it joined the candidate's path to the subpath and traversed the result from the root again, even though the traversal that collected the ancestors already holds each candidate's node. (The `start` argument the loop built for this was never read.) This lets the caller give either `opts.subpath`, which may be deep and behaves as before, or `opts.basename`, a single path segment. A basename is checked with one `Map.get` on the candidate's own node. It isn't validated to be a single segment - that'd be a cost on every call for something the caller already knows. With a basename, two cases still traverse: - A symlinked child, because we need to follow it to learn its type and real path. - Ancestors of the project root. Their nodes don't hold the segment that leads back down towards the root (with a root of `/A/B/C`, the node for `/A/B` has no `C` entry), so a map lookup there could wrongly report a directory as missing. Observations are recorded exactly as before. The existing test table now runs every row both as a `basename` and as a `subpath`, and the two must agree on the result and on the recorded sets. ### Compatibility The subpath moves from a positional argument into `opts`, so this changes the call shape for anything calling `hierarchicalLookup`. Metro's only caller passes `package.json`. Expo CLI's `createTypescriptResolver` passes deep subpaths (`node_modules/${targetPath}/tsconfig.json`) - that currently runs against `@expo/metro-file-map`'s own port of `TreeFS`, so it isn't affected, and deep subpaths remain supported here via `opts.subpath`. ## Benchmark Same harness as #1944 (6,935 resolutions, 7 interleaved rounds). Resolver output is byte-identical. | metric | #1945 | this diff | |---|---|---| | first pass after startup (ms) | 85.9 | **81.8 (−4.8%)** | | warm pass (ms) | 54.0 | 54.0 | | after a source change (ms) | 61.6 | 61.8 (+0.3%) | | after a `package.json` change (ms) | 64.4 | 63.7 (−1.1%) | Only the first pass moves, which is expected - after that these lookups are served by `PackageCache`. Isolating the walks themselves, by replaying the 4,483 closest-package walks of one pass directly against `hierarchicalLookup` (9 interleaved rounds): | | per walk | | |---|---|---| | #1945 | 865ns | | | this diff, `basename: 'package.json'` | **387ns** | **−55%** | | alternative: API unchanged, take the fast path when `subpath` has no separator | 394ns | −54% | The alternative is within noise of this, so the choice between them is about the API rather than speed. A deep subpath (`node_modules/react/package.json` from the same start paths) is 1,083ns -> 1,120ns per walk with either, about 3%, for the extra branch per candidate. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler/__tests__/resolver-test.js yarn flow check yarn typecheck-ts yarn verify-api-snapshots ``` New tests cover a directory-type lookup by basename, and a deep subpath. The existing fixture already covers a symlinked `package.json`, a broken link, and a directory named `package.json`. Changelog: Internal

…ith one map lookup Stacked on #1945. ## Summary `hierarchicalLookup` walks up from a start path asking whether each ancestor contains a given subpath - it's how `DependencyGraph` finds the closest `package.json`. For every candidate ancestor it joined the candidate's path to the subpath and traversed the result from the root again, even though the traversal that collected the ancestors already holds each candidate's node. (The `start` argument the loop built for this was never read.) When the subpath is a single path segment, which is the only way Metro calls it, each candidate is now checked with one `Map.get` on its own node. The API is unchanged, and a deep subpath behaves exactly as before. Whether the fast path applies is decided once per call, by checking the subpath for a path separator. With a single segment, two cases still traverse: - A symlinked child, because we need to follow it to learn its type and real path. - Ancestors of the project root. Their nodes don't hold the segment that leads back down towards the root (with a root of `/A/B/C`, the node for `/A/B` has no `C` entry), so a map lookup there could wrongly report a directory as missing. An empty subpath, `.` and `..` also take the traversal, since a map lookup would answer those differently. Observations are recorded exactly as before - the existing test table, which asserts the recorded sets, passes unchanged. I also tried the more explicit alternative of having the caller pass either a `basename` or a `subpath`, which avoids the separator check. It measured 387ns per walk against 394ns for this, within noise, and it changes the call shape for every caller - including Expo CLI's `createTypescriptResolver`, which passes deep subpaths - so it isn't worth it. ## Benchmark Same harness as #1944 (6,935 resolutions, 7 interleaved rounds). Resolver output is byte-identical. | metric | #1945 | this diff | |---|---|---| | first pass after startup (ms) | 85.2 | **82.2 (−3.5%)** | | warm pass (ms) | 53.3 | 54.8 | | after a source change (ms) | 61.1 | 62.3 | | after a `package.json` change (ms) | 64.2 | 64.2 | Only the first pass can move. The other passes make no `hierarchicalLookup` calls at all, because they're served by `PackageCache`, so those rows are run-to-run noise. Isolating the walks themselves, by replaying the 4,483 closest-package walks of one pass directly against `hierarchicalLookup`: 870ns -> 406ns per walk (−53%). A deep subpath (`node_modules/react/package.json` from the same start paths) is 1,084ns -> 1,111ns, for the extra branch per candidate. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler/__tests__/resolver-test.js yarn flow check yarn typecheck-ts yarn verify-api-snapshots ``` New tests cover a directory-type lookup with a single-segment subpath, and a deep subpath. The existing fixture already covers a symlinked `package.json`, a broken link, and a directory named `package.json`. Changelog: Internal

…ith one map lookup Stacked on #1945. ## Summary `hierarchicalLookup` walks up from a start path asking whether each ancestor contains a given subpath - it's how `DependencyGraph` finds the closest `package.json`. For every candidate ancestor it joined the candidate's path to the subpath and traversed the result from the root again, even though the traversal that collected the ancestors already holds each candidate's node. (The `start` argument the loop built for this was never read.) When the subpath is a single path segment, which is the only way Metro calls it, each candidate is now checked with one `Map.get` on its own node. The API is unchanged, and a deep subpath behaves exactly as before. Whether the fast path applies is decided once per call, by checking the subpath for a path separator. With a single segment, two cases still traverse: - A symlinked child, because we need to follow it to learn its type and real path. - Ancestors of the project root. Their nodes don't hold the segment that leads back down towards the root (with a root of `/A/B/C`, the node for `/A/B` has no `C` entry), so a map lookup there could wrongly report a directory as missing. An empty subpath, `.` and `..` also take the traversal, since a map lookup would answer those differently. Observations are recorded exactly as before - the existing test table, which asserts the recorded sets, passes unchanged. I also tried the more explicit alternative of having the caller pass either a `basename` or a `subpath`, which avoids the separator check. It measured 387ns per walk against 394ns for this, within noise, and it changes the call shape for every caller - including Expo CLI's `createTypescriptResolver`, which passes deep subpaths - so it isn't worth it. ## Benchmark Each closest-package walk is 2.1x faster, which takes 3.5% off first-pass resolution time on our benchmark app (described in the test plan). 7 interleaved rounds. Resolver output is byte-identical. | metric | #1945 | this diff | |---|---|---| | first pass after startup (ms) | 85.2 | **82.2 (−3.5%)** | | warm pass (ms) | 53.3 | 54.8 | | after a source change (ms) | 61.1 | 62.3 | | after a `package.json` change (ms) | 64.2 | 64.2 | Only the first pass can move. The other passes make no `hierarchicalLookup` calls at all, because they're served by `PackageCache`, so those rows are run-to-run noise. Isolating the walks themselves, by replaying the closest-package walks of one pass directly against `hierarchicalLookup`: 870ns -> 406ns per walk (2.1x). A deep subpath (`node_modules/react/package.json` from the same start paths) is 1,084ns -> 1,111ns, for the extra branch per candidate. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler/__tests__/resolver-test.js yarn flow check yarn typecheck-ts yarn verify-api-snapshots ``` New tests cover a directory-type lookup with a single-segment subpath, and a deep subpath. The existing fixture already covers a symlinked `package.json`, a broken link, and a directory named `package.json`. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the resolutions a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds. Changelog: Internal

Stacked on #1946. ## Summary `PackageCache` remembers the closest `package.json` for every module path it's asked about, with a reverse index from each `package.json` to the module paths under it and a set of paths known to have no package, both of which exist only to invalidate the first. `DependencyGraph` calls `invalidate` for every changed file to keep it honest. With the previous two PRs that memo no longer pays for itself. #1945 cut closest-package lookups 3.7x (64k -> 17k per build on our mid-size RN benchmark app*), and #1946 made each one a walk of single map lookups, 2.1x faster (870ns -> 406ns). This removes the memo and both indices, and looks up the closest package on every call. The parsed contents of each `package.json` are still cached per path, and `invalidate` now just drops those. The other reason to do this is incremental resolution (#1944). Any cache that sits between the resolver and the file system has to store the observations that produced each entry and replay them on a hit, or the second resolution to hit the cache can never be invalidated. That's a fair amount of machinery to add to a cache that's worth 3% of first-build resolution time. This only makes sense on top of the stack - at #1944, removing the memo costs 14% on a first pass, and nearly doubles a pass that re-resolves everything. \* Described in the test plan. ## Benchmark Replaying every `DependencyGraph.resolveDependency` call made by one iOS dev build of the benchmark app. This goes through `DependencyGraph`'s whole-resolution cache, which is cleared only by real file change events, so each row is something that actually happens in a session. 7 interleaved rounds, medians of 15 iterations. Resolver output is byte-identical. | scenario | #1946 | this diff | |---|---|---| | first build (ms) | 92.9 | 95.6 (+2.9%) | | rebuild, nothing changed (ms) | 7.58 | 7.61 | | after a source edit, re-resolving the edited module's 8 dependencies (µs) | 155 | 155 | | after a `package.json` change, re-resolving everything (ms) | 73.6 | 80.4 (+9.3%) | | heap retained after resolving (MB) | 3.0 | **1.9 (−37%)** | The tradeoffs are: - ~3% of resolution time on a first build (3ms here). - ~9% (7ms here) when a `package.json` or symlink change resets the whole graph, which is the only time everything is re-resolved against a warm `PackageCache`. Incremental resolution aims to remove those full resets. - Nothing on an ordinary edit. Metro re-resolves only the edited module's dependencies, and the difference there is within noise. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler packages/metro/src/integration_tests yarn flow check yarn verify-api-snapshots ``` `PackageCache-test.js` is restructured around the new contract: the closest package is looked up on every call, a change of closest package is reflected with no invalidation, each `package.json` is parsed once however many modules share it, and invalidating one re-reads it. Tests of the reverse index and the no-package set go with those structures. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the `resolveDependency` calls a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds, with source and `package.json` edits made on disk and picked up by the watcher. Changelog: Internal

…ith one map lookup Stacked on #1945. ## Summary `hierarchicalLookup` walks up from a start path asking whether each ancestor contains a given subpath - it's how `DependencyGraph` finds the closest `package.json`. For every candidate ancestor it joined the candidate's path to the subpath and traversed the result from the root again, even though the traversal that collected the ancestors already holds each candidate's node. (The `start` argument the loop built for this was never read.) When the subpath is a single path segment, which is the only way Metro calls it, each candidate is now checked with one `Map.get` on its own node. The API is unchanged, and a deep subpath behaves exactly as before. Whether the fast path applies is decided once per call, by checking the subpath for a path separator. With a single segment, two cases still traverse: - A symlinked child, because we need to follow it to learn its type and real path. - Ancestors of the project root. Their nodes don't hold the segment that leads back down towards the root (with a root of `/A/B/C`, the node for `/A/B` has no `C` entry), so a map lookup there could wrongly report a directory as missing. An empty subpath, `.` and `..` also take the traversal, since a map lookup would answer those differently. Observations are recorded exactly as before - the existing test table, which asserts the recorded sets, passes unchanged. I also tried the more explicit alternative of having the caller pass either a `basename` or a `subpath`, which avoids the separator check. It measured 387ns per walk against 394ns for this, within noise, and it changes the call shape for every caller - including Expo CLI's `createTypescriptResolver`, which passes deep subpaths - so it isn't worth it. ## Benchmark Each closest-package walk is 2.1x faster, which takes 3.5% off first-pass resolution time, measured in isolation on our benchmark app (described in the test plan). 7 interleaved rounds. Resolver output is byte-identical. | metric | #1945 | this diff | |---|---|---| | first pass after startup (ms) | 85.2 | **82.2 (−3.5%)** | | warm pass (ms) | 53.3 | 54.8 | | after a source change (ms) | 61.1 | 62.3 | | after a `package.json` change (ms) | 64.2 | 64.2 | Only the first pass can move. The other passes make no `hierarchicalLookup` calls at all, because they're served by `PackageCache`, so those rows are run-to-run noise. Isolating the walks themselves, by replaying the closest-package walks of one pass directly against `hierarchicalLookup`: 870ns -> 406ns per walk (2.1x). A deep subpath (`node_modules/react/package.json` from the same start paths) is 1,084ns -> 1,111ns, for the extra branch per candidate. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler/__tests__/resolver-test.js yarn flow check yarn typecheck-ts yarn verify-api-snapshots ``` New tests cover a directory-type lookup with a single-segment subpath, and a deep subpath. The existing fixture already covers a symlinked `package.json`, a broken link, and a directory named `package.json`. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the resolutions a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it. Changelog: Internal

Stacked on #1946. ## Summary `PackageCache` remembers the closest `package.json` for every module path it's asked about, with a reverse index from each `package.json` to the module paths under it and a set of paths known to have no package, both of which exist only to invalidate the first. `DependencyGraph` calls `invalidate` for every changed file to keep it honest. With the previous two PRs that memo no longer pays for itself. #1945 cut closest-package lookups 3.7x (64k -> 17k per build on our mid-size RN benchmark app*), and #1946 made each one a walk of single map lookups, 2.1x faster (870ns -> 406ns). This removes the memo and both indices, and looks up the closest package on every call. The parsed contents of each `package.json` are still cached per path, and `invalidate` now just drops those. The other reason to do this is incremental resolution (#1944). Any cache that sits between the resolver and the file system has to store the observations that produced each entry and replay them on a hit, or the second resolution to hit the cache can never be invalidated. That's a fair amount of machinery to add to a cache that's worth 3% of first-build resolution time, which is itself a small part of a build. This only makes sense on top of the stack - at #1944, removing the memo costs 14% on a first pass, and nearly doubles a pass that re-resolves everything. \* Described in the test plan. ## Benchmark Resolution time in isolation, replaying every `DependencyGraph.resolveDependency` call made by one iOS dev build of the benchmark app. Nothing is transformed or serialised, so these are percentages of resolution, not of a build. This goes through `DependencyGraph`'s whole-resolution cache, which is cleared only by real file change events, so each row is something that actually happens in a session. 7 interleaved rounds, medians of 15 iterations. Resolver output is byte-identical. | scenario | #1946 | this diff | |---|---|---| | first build (ms) | 92.9 | 95.6 (+2.9%) | | rebuild, nothing changed (ms) | 7.58 | 7.61 | | after a source edit, re-resolving the edited module's 8 dependencies (µs) | 155 | 155 | | after a `package.json` change, re-resolving everything (ms) | 73.6 | 80.4 (+9.3%) | | heap retained after resolving (MB) | 3.0 | **1.9 (−37%)** | The tradeoffs are: - ~3% of resolution time on a first build (3ms here). A real build does far more than resolve - the same build takes around 4s end to end with a warm transform cache - so that's under 0.1% of it. - ~9% (7ms here) when a `package.json` or symlink change resets the whole graph, which is the only time everything is re-resolved against a warm `PackageCache`. Incremental resolution aims to remove those full resets. - Nothing on an ordinary edit. Metro re-resolves only the edited module's dependencies, and the difference there is within noise. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler packages/metro/src/integration_tests yarn flow check yarn verify-api-snapshots ``` `PackageCache-test.js` is restructured around the new contract: the closest package is looked up on every call, a change of closest package is reflected with no invalidation, each `package.json` is parsed once however many modules share it, and invalidating one re-reads it. Tests of the reverse index and the no-package set go with those structures. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the `resolveDependency` calls a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds, with source and `package.json` edits made on disk and picked up by the watcher. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it. Changelog: Internal

Stacked on #1946. ## Summary `PackageCache` remembers the closest `package.json` for every module path it's asked about, with a reverse index from each `package.json` to the module paths under it and a set of paths known to have no package, both of which exist only to invalidate the first. `DependencyGraph` calls `invalidate` for every changed file to keep it honest. With the previous two PRs that memo no longer pays for itself. #1945 cut closest-package lookups 3.7x (64k -> 17k per build on our mid-size RN benchmark app*), and #1946 made each one a walk of single map lookups, 2.1x faster (870ns -> 406ns). This removes the memo and both indices, and looks up the closest package on every call. The parsed contents of each `package.json` are still cached per path, and `invalidate` now just drops those. The other reason to do this is incremental resolution (#1944). Any cache that sits between the resolver and the file system has to store the observations that produced each entry and replay them on a hit, or the second resolution to hit the cache can never be invalidated. That's a fair amount of machinery to add to a cache that's worth 3% of first-build resolution time, which is itself a small part of a build. This only makes sense on top of the stack - at #1944, removing the memo costs 14% on a first pass, and nearly doubles a pass that re-resolves everything. \* Described in the test plan. ## Bugs fixed This also fixes two invalidation bugs, both present on main, where the watcher picks up a change on disk but resolution keeps its old answer until Metro restarts: - A nearer `package.json` is added, e.g. creating a nested package in a monorepo. The memo is invalidated through a reverse index from each `package.json` to the modules that resolved to it, and a new `package.json` has no entries, so the modules beneath it keep pointing at the farther one. (It's masked when there was previously no enclosing package at all, which is special-cased.) - A directory symlink is retargeted into another package. Entries are keyed by the path as it was asked for, through the link, and only the link's own path is invalidated. Both changes reset the graph, so everything is re-resolved - against the stale memo, which is probably why they've gone unnoticed. Reproduced with a real `DependencyGraph` and watcher on main and on #1946, using a `browser` redirect in the affected `package.json` to show which one was consulted. With no memo there's nothing to go stale - every lookup walks the live file map. ## Benchmark Resolution time in isolation, replaying every `DependencyGraph.resolveDependency` call made by one iOS dev build of the benchmark app. Nothing is transformed or serialised, so these are percentages of resolution, not of a build. This goes through `DependencyGraph`'s whole-resolution cache, which is cleared only by real file change events, so each row is something that actually happens in a session. 7 interleaved rounds, medians of 15 iterations. Resolver output is byte-identical. | scenario | #1946 | this diff | |---|---|---| | first build (ms) | 92.9 | 95.6 (+2.9%) | | rebuild, nothing changed (ms) | 7.58 | 7.61 | | after a source edit, re-resolving the edited module's 8 dependencies (µs) | 155 | 155 | | after a `package.json` change, re-resolving everything (ms) | 73.6 | 80.4 (+9.3%) | | heap retained after resolving (MB) | 3.0 | **1.9 (−37%)** | The tradeoffs are: - ~3% of resolution time on a first build (3ms here). A real build does far more than resolve - the same build takes around 4s end to end with a warm transform cache - so that's under 0.1% of it. - ~9% (7ms here) when a `package.json` or symlink change resets the whole graph, which is the only time everything is re-resolved against a warm `PackageCache`. Incremental resolution aims to remove those full resets. - Nothing on an ordinary edit. Metro re-resolves only the edited module's dependencies, and the difference there is within noise. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler packages/metro/src/integration_tests yarn flow check yarn verify-api-snapshots ``` `PackageCache-test.js` is restructured around the new contract: the closest package is looked up on every call, a change of closest package is reflected with no invalidation, each `package.json` is parsed once however many modules share it, and invalidating one re-reads it. Tests of the reverse index and the no-package set go with those structures. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the `resolveDependency` calls a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds, with source and `package.json` edits made on disk and picked up by the watcher. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it. Changelog: ``` - **[Fix]**: Resolver: Fix stale resolutions after adding a nearer `package.json` or retargeting a symlinked directory, which previously needed a restart ```

Stacked on #1946. ## Summary `PackageCache` remembers the closest `package.json` for every module path it's asked about, with a reverse index from each `package.json` to the module paths under it and a set of paths known to have no package, both of which exist only to invalidate the first. `DependencyGraph` calls `invalidate` for every changed file to keep it honest. With the previous two PRs that memo no longer pays for itself. #1945 cut closest-package lookups 3.7x (64k -> 17k per build on our mid-size RN benchmark app*), and #1946 made each one a walk of single map lookups, 2.1x faster (870ns -> 406ns). This removes the memo and both indices, and looks up the closest package on every call. The parsed contents of each `package.json` are still cached per path, and `invalidate` now just drops those. The other reason to do this is incremental resolution (#1944). Any cache that sits between the resolver and the file system has to store the observations that produced each entry and replay them on a hit, or the second resolution to hit the cache can never be invalidated. That's a fair amount of machinery to add to a cache that's worth 3% of first-build resolution time, which is itself a small part of a build. This only makes sense on top of the stack - at #1944, removing the memo costs 14% on a first pass, and nearly doubles a pass that re-resolves everything. \* Described in the test plan. ## Bugs fixed This also fixes two invalidation bugs, both present on main, where the watcher picks up a change on disk but resolution keeps its old answer until Metro restarts: - A nearer `package.json` is added, e.g. creating a nested package in a monorepo. The memo is invalidated through a reverse index from each `package.json` to the modules that resolved to it, and a new `package.json` has no entries, so the modules beneath it keep pointing at the farther one. (It's masked when there was previously no enclosing package at all, which is special-cased.) - A directory symlink is retargeted into another package. Entries are keyed by the path as it was asked for, through the link, and only the link's own path is invalidated. Both changes reset the graph, so everything is re-resolved - against the stale memo, which is probably why they've gone unnoticed. Reproduced with a real `DependencyGraph` and watcher on main and on #1946, using a `browser` redirect in the affected `package.json` to show which one was consulted. With no memo there's nothing to go stale - every lookup walks the live file map. ## Benchmark Resolution time in isolation, replaying every `DependencyGraph.resolveDependency` call made by one iOS dev build of the benchmark app. Nothing is transformed or serialised, so these are percentages of resolution, not of a build. This goes through `DependencyGraph`'s whole-resolution cache, which is cleared only by real file change events, so each row is something that actually happens in a session. 7 interleaved rounds, medians of 15 iterations. Resolver output is byte-identical. | scenario | #1946 | this diff | |---|---|---| | first build (ms) | 92.9 | 95.6 (+2.9%) | | rebuild, nothing changed (ms) | 7.58 | 7.61 | | after a source edit, re-resolving the edited module's 8 dependencies (µs) | 155 | 155 | | after a `package.json` change, re-resolving everything (ms) | 73.6 | 80.4 (+9.3%) | | heap retained after resolving (MB) | 3.0 | **1.9 (−37%)** | The tradeoffs are: - ~3% of resolution time on a first build (3ms here). A real build does far more than resolve - the same build takes around 4s end to end with a warm transform cache - so that's under 0.1% of it. - ~9% (7ms here) when a `package.json` or symlink change resets the whole graph, which is the only time everything is re-resolved against a warm `PackageCache`. Incremental resolution aims to remove those full resets. - Nothing on an ordinary edit. Metro re-resolves only the edited module's dependencies, and the difference there is within noise. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler packages/metro/src/integration_tests yarn flow check yarn verify-api-snapshots ``` `PackageCache-test.js` loses its tests of memo invalidation, along with the structures they tested - with no memo, invalidation is a non-issue by construction. What's left covers the one remaining cache: each `package.json` is parsed once however many modules share it, and is re-read after it's invalidated. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the `resolveDependency` calls a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds, with source and `package.json` edits made on disk and picked up by the watcher. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it. Changelog: ``` - **[Fix]**: Resolver: Fix stale resolutions after adding a nearer `package.json` or retargeting a symlinked directory, which previously needed a restart ```

Stacked on #1945. `hierarchicalLookup` walks up from a start path asking whether each ancestor contains a given subpath - it's how `DependencyGraph` finds the closest `package.json`. For every candidate ancestor it joined the candidate's path to the subpath and traversed the result from the root again, even though the traversal that collected the ancestors already holds each candidate's node. The loop even built a `start` argument for `#lookupByNormalPath` to resume from, but it was never read. Each candidate is now checked from its own node: - When the subpath is a single path segment, which is the only way Metro calls it, with one `Map.get`. Whether that applies is decided once per call, by checking the subpath for a path separator. - Otherwise, by traversing only the subpath from the candidate. Expo CLI's `createTypescriptResolver` calls it this way, with deep subpaths such as `node_modules/${targetPath}/tsconfig.json`. Some single segments still take the traversal from the candidate: - A symlinked child, because we need to follow it to learn its type and real path. - A segment on an ancestor of the project root. Its node doesn't hold the segment that leads back down towards the root (with a root of `/A/B/C`, the node for `/A/B` has no `C` entry), so a map lookup there could wrongly report a directory as missing. A subpath that re-enters the project root from an ancestor of it, e.g. `B/C/a/package.json` from `/A`, is still traversed from the root. The joined path collapses to `a/package.json`, of which the candidate's path is no longer a prefix. An empty subpath, `.` and `..` also take the traversal, since a map lookup would answer those differently. The API is unchanged, and observations are recorded exactly as before - the existing test table, which asserts the recorded sets, passes unchanged. This also removes the unused `startPathIdx` and `startNode` options of `#lookupByNormalPath`. I also tried the more explicit alternative of having the caller pass either a `basename` or a `subpath`, which avoids the separator check. It measured 387ns per walk against 394ns for the separator check, within noise, and it changes the call shape for every caller, so it isn't worth it. ## Benchmark Each closest-package walk is 2.1x faster and each deep-subpath walk 1.4x faster, which takes 13% off first-pass resolution time against `main` on our benchmark app (described in the test plan). Resolver output is byte-identical. ### Walks Replaying the closest-package walks of one resolver pass directly against `hierarchicalLookup` (4,483 walks on the benchmark app), once for `package.json` and once for a deep subpath (`node_modules/react/package.json`) from the same start paths. Nanoseconds per walk, medians of 9 interleaved rounds. The middle rows are each half of this diff on its own, to show which half does what: | variant | `package.json` | deep subpath | |---|---|---| | `main` | 911 | 1,150 | | traversal from the candidate only | 598 (−34%) | 792 (−31%) | | `Map.get` for a single segment only | 426 (−53%) | 1,161 (+1%) | | this diff | **426 (−53%)** | **813 (−29%)** | The two halves are complementary: the map lookup is the bigger win for a single segment, and only the traversal from the candidate helps a deep subpath. Together they cost nothing on either path - the ~20ns behind "traversal only" on a deep subpath is the separator check. ### Resolver, end to end Resolution in isolation, 7 interleaved rounds. Only the first pass can move - the later passes are served by `PackageCache` and make no `hierarchicalLookup` calls, and are within ±2% of `main`. | metric | `main` | this diff | |---|---|---| | first pass after startup (ms) | 125.3 | **109.2 (−12.9%)** | ## Changelog Internal ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler/__tests__/resolver-test.js yarn flow check yarn typecheck-ts yarn verify-api-snapshots ``` New tests cover a directory-type lookup with a single-segment subpath, a deep subpath, and a subpath that re-enters the project root from an ancestor of it - that last one fails if traversal always starts from the candidate. The existing fixture already covers a symlinked `package.json`, a broken link, and a directory named `package.json`. **Benchmarks** - measured with this diff applied directly to `main` (at a95869a) rather than on top of #1945, so that the baseline is `main`. The ports substitute `main`'s `invalidatedBy` for #1944's observations, and give identical results for every walk, including the recorded paths. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the resolutions a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it.

Stacked on #1946. ## Summary `PackageCache` remembers the closest `package.json` for every module path it's asked about, with a reverse index from each `package.json` to the module paths under it and a set of paths known to have no package, both of which exist only to invalidate the first. `DependencyGraph` calls `invalidate` for every changed file to keep it honest. With the previous two PRs that memo no longer pays for itself. #1945 cut closest-package lookups 3.7x (64k -> 17k per build on our mid-size RN benchmark app*), and #1946 made each one a walk of single map lookups, 2.1x faster (870ns -> 406ns). This removes the memo and both indices, and looks up the closest package on every call. The parsed contents of each `package.json` are still cached per path, and `invalidate` now just drops those. The other reason to do this is incremental resolution (#1944). Any cache that sits between the resolver and the file system has to store the observations that produced each entry and replay them on a hit, or the second resolution to hit the cache can never be invalidated. That's a fair amount of machinery to add to a cache that's worth 3% of first-build resolution time, which is itself a small part of a build. This only makes sense on top of the stack - at #1944, removing the memo costs 14% on a first pass, and nearly doubles a pass that re-resolves everything. \* Described in the test plan. ## Bugs fixed This also fixes two invalidation bugs, both present on main, where the watcher picks up a change on disk but resolution keeps its old answer until Metro restarts: - A nearer `package.json` is added, e.g. creating a nested package in a monorepo. The memo is invalidated through a reverse index from each `package.json` to the modules that resolved to it, and a new `package.json` has no entries, so the modules beneath it keep pointing at the farther one. (It's masked when there was previously no enclosing package at all, which is special-cased.) - A directory symlink is retargeted into another package. Entries are keyed by the path as it was asked for, through the link, and only the link's own path is invalidated. Both changes reset the graph, so everything is re-resolved - against the stale memo, which is probably why they've gone unnoticed. Reproduced with a real `DependencyGraph` and watcher on main and on #1946, using a `browser` redirect in the affected `package.json` to show which one was consulted. With no memo there's nothing to go stale - every lookup walks the live file map. ## Benchmark Resolution time in isolation, replaying every `DependencyGraph.resolveDependency` call made by one iOS dev build of the benchmark app. Nothing is transformed or serialised, so these are percentages of resolution, not of a build. This goes through `DependencyGraph`'s whole-resolution cache, which is cleared only by real file change events, so each row is something that actually happens in a session. 7 interleaved rounds, medians of 15 iterations. Resolver output is byte-identical. | scenario | #1946 | this diff | |---|---|---| | first build (ms) | 92.9 | 95.6 (+2.9%) | | rebuild, nothing changed (ms) | 7.58 | 7.61 | | after a source edit, re-resolving the edited module's 8 dependencies (µs) | 155 | 155 | | after a `package.json` change, re-resolving everything (ms) | 73.6 | 80.4 (+9.3%) | | heap retained after resolving (MB) | 3.0 | **1.9 (−37%)** | The tradeoffs are: - ~3% of resolution time on a first build (3ms here). A real build does far more than resolve - the same build takes around 4s end to end with a warm transform cache - so that's under 0.1% of it. - ~9% (7ms here) when a `package.json` or symlink change resets the whole graph, which is the only time everything is re-resolved against a warm `PackageCache`. Incremental resolution aims to remove those full resets. - Nothing on an ordinary edit. Metro re-resolves only the edited module's dependencies, and the difference there is within noise. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler packages/metro/src/integration_tests yarn flow check yarn verify-api-snapshots ``` `PackageCache-test.js` loses its tests of memo invalidation, along with the structures they tested - with no memo, invalidation is a non-issue by construction. What's left covers the one remaining cache: each `package.json` is parsed once however many modules share it, and is re-read after it's invalidated. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the `resolveDependency` calls a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds, with source and `package.json` edits made on disk and picked up by the watcher. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it. Changelog: ``` - **[Fix]**: Resolver: Fix stale resolutions after adding a nearer `package.json` or retargeting a symlinked directory, which previously needed a restart ```

Copilot

🟡 Changes recommended

The lexical node_modules shortcut changes package redirection behavior for symlinked directories.

3 open findings

Resolve symlinked node_modules before applying package browser mappings · New
Describe test-cache isolation without citing obsolete TreeFS behavior · New
Update rationale to reflect cross-realm TreeFS map support · New

What changed in this PR

Refactors resolver package-scope lookup to reuse one directory lookup across extension candidates, improving resolution performance.

Changes:

Shares package scope across source-file candidates.
Extracts package-subpath redirection logic.
Adds lookup-count tests and resets test caches.

File	Description
`packages/metro-resolver/src/resolve.js`	Reuses directory package scope.
`packages/metro-resolver/src/PackageResolve.js`	Extracts redirection helper.
`packages/metro-resolver/src/types.js`	Clarifies package lookup semantics.
`packages/metro-resolver/src/__tests__/package-scope-test.js`	Tests package-scope lookup counts.
`packages/metro/src/DeltaBundler/__tests__/resolver-test.js`	Resets test file-map cache.
`packages/metro/src/integration_tests/metro.config.js`	Resets integration-test cache.

🧠 Review effort: Balanced

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Copilot

🟡 Changes recommended

Leaf symlinks can resolve under the wrong package scope, and valid stand-in-name collisions can crash resolution.

4 open findings

1 resolved since last review

Resolve symlinked node_modules before applying package browser mappings

🧠 Review effort: Balanced

robhogan · 2026-10-19T07:10:30Z

+    ...context,
+    candidateExts,
+    filePathPrefix,
+    candidatePackageScope: getCandidatePackageScope(context, dirPath, fileName),


A leaf symlink candidate was never scoped by its target. For a path that ends in a file symlink, hierarchicalLookup walks the ancestors of the link, not those of its target, so the per-candidate lookup on main never applied the target package's browser map either. The output is unchanged. I've added two resolver-test.js cases that pin this: "a file symlink into another package, mapped by its target package" (the target's map isn't applied) and "...mapped by the package of the link" (the link's package's map is). Both pass with main's resolver as well.

robhogan · 2026-10-19T07:10:31Z

+    if (attempt === MAX_ANY_FILE_NAME_ATTEMPTS) {
+      throw new Error(
+        `getPackageForModule(${JSON.stringify(modulePath)}) returned ` +
+          `packageRelativePath ${JSON.stringify(packageRelativePath)}, ` +
+          `which should end in ${JSON.stringify(name)}.`,


The stand-in is now the directory's package.json. It only collides when package.json is a directory, or a link to one, containing another package.json.

To compensate for that edge case we detect that and try package.json_ and package.json__, before throwing in the case that all three are directories containing package.json, which is obviously vanishingly unlikely.

## Summary `redirectModulePath` asks `context.getPackageForModule` for the package containing a module path, then applies that package's `mainFields` redirections (the "browser" spec) to the module's package-relative path. This splits the second half out as `redirectPackageSubpath`, for callers that already know the containing package. `redirectModulePath` delegates to it for relative and absolute paths and behaves exactly as before; bare specifiers are unchanged. #1945 uses it to look up the package of a directory's files once and apply it to every extension candidate, instead of calling `redirectModulePath` per candidate. ## Test plan ``` yarn jest packages/metro-resolver packages/metro/src/DeltaBundler/__tests__/resolver-test.js yarn flow check ``` Changelog: Internal

Stacked on #1946. ## Summary `PackageCache` remembers the closest `package.json` for every module path it's asked about, with a reverse index from each `package.json` to the module paths under it and a set of paths known to have no package, both of which exist only to invalidate the first. `DependencyGraph` calls `invalidate` for every changed file to keep it honest. With the previous two PRs that memo no longer pays for itself. #1945 cut closest-package lookups 3.7x (64k -> 17k per build on our mid-size RN benchmark app*), and #1946 made each one a walk of single map lookups, 2.1x faster (870ns -> 406ns). This removes the memo and both indices, and looks up the closest package on every call. The parsed contents of each `package.json` are still cached per path, and `invalidate` now just drops those. The other reason to do this is incremental resolution (#1944). Any cache that sits between the resolver and the file system has to store the observations that produced each entry and replay them on a hit, or the second resolution to hit the cache can never be invalidated. That's a fair amount of machinery to add to a cache that's worth 3% of first-build resolution time, which is itself a small part of a build. This only makes sense on top of the stack - at #1944, removing the memo costs 14% on a first pass, and nearly doubles a pass that re-resolves everything. \* Described in the test plan. ## Bugs fixed This also fixes two invalidation bugs, both present on main, where the watcher picks up a change on disk but resolution keeps its old answer until Metro restarts: - A nearer `package.json` is added, e.g. creating a nested package in a monorepo. The memo is invalidated through a reverse index from each `package.json` to the modules that resolved to it, and a new `package.json` has no entries, so the modules beneath it keep pointing at the farther one. (It's masked when there was previously no enclosing package at all, which is special-cased.) - A directory symlink is retargeted into another package. Entries are keyed by the path as it was asked for, through the link, and only the link's own path is invalidated. Both changes reset the graph, so everything is re-resolved - against the stale memo, which is probably why they've gone unnoticed. Reproduced with a real `DependencyGraph` and watcher on main and on #1946, using a `browser` redirect in the affected `package.json` to show which one was consulted. With no memo there's nothing to go stale - every lookup walks the live file map. ## Benchmark Resolution time in isolation, replaying every `DependencyGraph.resolveDependency` call made by one iOS dev build of the benchmark app. Nothing is transformed or serialised, so these are percentages of resolution, not of a build. This goes through `DependencyGraph`'s whole-resolution cache, which is cleared only by real file change events, so each row is something that actually happens in a session. 7 interleaved rounds, medians of 15 iterations. Resolver output is byte-identical. | scenario | #1946 | this diff | |---|---|---| | first build (ms) | 92.9 | 95.6 (+2.9%) | | rebuild, nothing changed (ms) | 7.58 | 7.61 | | after a source edit, re-resolving the edited module's 8 dependencies (µs) | 155 | 155 | | after a `package.json` change, re-resolving everything (ms) | 73.6 | 80.4 (+9.3%) | | heap retained after resolving (MB) | 3.0 | **1.9 (−37%)** | The tradeoffs are: - ~3% of resolution time on a first build (3ms here). A real build does far more than resolve - the same build takes around 4s end to end with a warm transform cache - so that's under 0.1% of it. - ~9% (7ms here) when a `package.json` or symlink change resets the whole graph, which is the only time everything is re-resolved against a warm `PackageCache`. Incremental resolution aims to remove those full resets. - Nothing on an ordinary edit. Metro re-resolves only the edited module's dependencies, and the difference there is within noise. ## Test plan ``` yarn jest packages/metro-file-map packages/metro-resolver packages/metro/src/node-haste packages/metro/src/DeltaBundler packages/metro/src/integration_tests yarn flow check yarn verify-api-snapshots ``` `PackageCache-test.js` loses its tests of memo invalidation, along with the structures they tested - with no memo, invalidation is a non-issue by construction. What's left covers the one remaining cache: each `package.json` is parsed once however many modules share it, and is re-read after it's invalidated. **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the `resolveDependency` calls a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds, with source and `package.json` edits made on disk and picked up by the watcher. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it. Changelog: ``` - **[Fix]**: Resolver: Fix stale resolutions after adding a nearer `package.json` or retargeting a symlinked directory, which previously needed a restart ```

…andidates ## Summary `resolveFile` tries each candidate extension in turn, and for each one `resolveSourceFileForExt` calls `redirectModulePath`, which asks `context.getPackageForModule` for the package scope of the candidate path. Every candidate in a directory belongs to the same package, so that's the same question asked once per extension - and because Metro's `PackageCache` memoises by module path, each distinct candidate string misses the first time it's seen, mostly for files that don't exist. This asks once, up front, for the package of the files in the directory, and shares it with every candidate via the source file context, applying each candidate's `mainFields` redirections with `redirectPackageSubpath`. The package of the files in a directory isn't always the package of the directory itself - a `node_modules` directory belongs to whatever contains it while the files in it belong to none, and a symlink can change which directory along the way is named `node_modules`. So `getCandidatePackageScope` asks about the directory's `package.json`, which every lookup for a file there checks, so it gets the same answer any real candidate there would. If `package.json` is instead a directory that is itself a package root, the answer is about that package, which shows in its package-relative path, so it tries another name, and throws if `getPackageForModule` keeps answering about something else. `package-scope-test.js` pins the sequence of `getPackageForModule` calls the resolver makes for the common specifier shapes, so that the call count is a tested property rather than incidental. Four `resolver-test.js` cases pin the package scope of files reached through a symlink, which is that of where the link is: to or from a `node_modules` directory, where the directory and the files in it disagree, and a file symlink into another package. This also documents what `getPackageForModule` returns for symlinked and missing paths. ## Benchmark This cuts package scope lookups 3.6x, and takes 33% off first-pass resolution time, measured in isolation on our benchmark app (described in the test plan). 7 interleaved rounds. Resolver output is byte-identical. | metric | main | this diff | |---|---|---| | `getPackageForModule` calls per pass | 64k | **18k (3.6x fewer)** | | first pass after startup (ms) | 130.4 | **87.3 (−33.1%)** | | warm pass (ms) | 57.3 | **54.7 (−4.5%)** | | after a source change (ms) | 61.0 | **59.4 (−2.6%)** | | after a `package.json` change (ms) | 65.9 | **60.7 (−7.8%)** | The first pass is where this lands, because that's where every distinct candidate path is a `PackageCache` miss and a walk up the tree. ## Test plan ``` yarn jest packages/metro-resolver packages/metro-file-map packages/metro/src/node-haste packages/metro/src/DeltaBundler/__tests__/resolver-test.js yarn flow check yarn verify-api-snapshots ``` **Benchmark app** - a synthetic mid-size RN 0.87 app: 23 common dependencies (Reanimated, React Navigation, TanStack Query, lodash, etc.) and ~1,000 generated first-party modules, 60% of them in a workspace package outside `projectRoot` with its own `node_modules`. The iOS dev bundle is 2.8k modules, from a file map of 34k files. Timings replay the resolutions a real build of it makes against a fresh `DependencyGraph` per process, in interleaved rounds. These timings isolate resolution - no transformation, serialisation or file reads are included. A real build of this app takes around 4s even with a warm transform cache, so resolution is about 2% of it. Changelog: Internal

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metro-file-map: Check hierarchicalLookup candidates from their own nodes #1946

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Remove PackageCache's per-module closest package memo #1948

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This was referenced Sep 26, 2026

Resolver: Collect file system observations per resolution under unstable_incrementalResolution #1949

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Resolver: Observe Haste names under unstable_incrementalResolution #1952

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Resolver: Extract redirectPackageSubpath from redirectModulePath #2044

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Base automatically changed from robhogan/redirect-package-subpath to main October 14, 2026 10:11

robhogan force-pushed the robhogan/resolve-package-once-per-directory branch from 7e3174d to 56dedec Compare October 14, 2026 10:11

robhogan force-pushed the robhogan/resolve-package-once-per-directory branch 3 times, most recently from 8f22368 to 06b6825 Compare October 19, 2026 07:34

robhogan marked this pull request as ready for review October 19, 2026 07:48

robhogan force-pushed the robhogan/resolve-package-once-per-directory branch from 06b6825 to 638f35c Compare October 19, 2026 07:48

robhogan requested review from huntie and vzaidman October 19, 2026 07:48

facebook-github-tools Bot added the Shared with Meta Applied via automation to indicate that an Issue or Pull Request has been shared with the team. label Oct 19, 2026

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Resolver: Resolve the package of a directory once for all extension candidates - #1945

Resolver: Resolve the package of a directory once for all extension candidates#1945
robhogan wants to merge 1 commit into
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robhogan commented Sep 18, 2026 •

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robhogan Oct 19, 2026 •

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	function resolveSourceFileForExt(
	context: SourceFileContext,
	extension: string,
	): SourceFileResolution {
	const filePath = `${context.filePathPrefix}${extension}`;
	const redirectedPath =
	// Any redirections for the bare path have already happened
	extension !== '' ? redirectModulePath(context, filePath) : filePath;

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robhogan commented Sep 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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robhogan Oct 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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robhogan commented Sep 18, 2026 •

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robhogan Oct 19, 2026 •

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