Resolver: Remaining differences from Node in `"exports"` resolution

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
42/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
javascript
Ambito
backend, tooling

Direzione di ricerca

Start in packages/metro-resolver/src/PackageExportsResolve.js, packages/metro-resolver/src/utils/matchSubpathPattern.js, packages/metro-resolver/src/utils/matchSubpathFromExportsLike.js, and packages/metro-file-map/src/lib/TreeFS.js, then compare the listed cases against Node’s require.resolve. The issue identifies several concrete mismatches, but does not name tests to run. Done means the applicable cases produce Node-compatible resolution results or Metro resolution errors instead of raw exceptions or silent incorrect resolutions.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

With #2160, Metro follows Node's PACKAGE_RESOLVE for which package.json and subpath to use with "exports". A comparison with Node (AI-driven, against require.resolve on Node 26, and on Node 24 for Windows) found these remaining differences. Unless noted, they reproduce with resolver.unstable_resolveExportsFromPackageRoot on or off, so they aren't new with that flag.

I've left out Metro's warn-and-fall-back for unexported subpaths and invalid configs, which is intentional for now, and pkg/ resolving as pkg where there's a "." entry (Node says not exported), which we're keeping because require('x/') is a common idiom.

None of these look common. I'd start with the TypeErrors, since those fail on packages that work in Node.

Metro fails where Node resolves

Case Specifier Node Metro Code
"exports": false (or true, 1), with "main": "m.js" pkg m.js Raw TypeError Non-object WeakMap key
package.json is a symlink to store/manifest.json, with "exports": {".": "./root.js"} and root.js beside the symlink pkg node_modules/pkg/root.js Warns about a missing file, fails to resolve Targets are relative to the manifest's real directory
No "exports", "main": "lib/m.js" pkg/. lib/m.js Raw Error: Unexpectedly escaped traversal TreeFS invariant
No root package.json, sub/package.json has "main" pkg//sub sub/main.js Fails to resolve
missing doesn't exist, or @m has no package missing/../pkg/a.js, @m/../pkg/a.js pkg/a.js under require Fails to resolve

pkg/. also throws that raw Error for a package with "exports". Node rejects those too, but we should fail with a resolution error.

Metro resolves where Node rejects

These are silent, with no warning.

Case Specifier Node Metro Code
"exports": {"./lib*": "./dist/lib*.js"} pkg/lib ERR_PACKAGE_PATH_NOT_EXPORTED dist/lib.js No length check without a trailer
"exports": {"./a/": "./ok.js"} pkg/a/ ERR_PACKAGE_PATH_NOT_EXPORTED ok.js Exact match on a key ending in /
"exports": {"./a": "./NODE_MODULES/a.js"} pkg/a ERR_INVALID_PACKAGE_TARGET NODE_MODULES/a.js Case-sensitive segment check
"exports": {"./a": {"0": "./x.js", "default": "./y.js"}} pkg/a ERR_INVALID_PACKAGE_CONFIG y.js Numeric condition keys aren't rejected
"exports": {"./a": "./x.js", "#b": "./y.js"} pkg/a ERR_INVALID_PACKAGE_CONFIG x.js # keys count as subpaths

Other

  • Percent-encoding: Node decodes pkg/%2e%2e/secret.js and throws ERR_INVALID_MODULE_SPECIFIER, and it rejects a target like ./%2e%2e/b.js with ERR_INVALID_PACKAGE_TARGET. Metro treats these as literal paths, so they warn about a missing file and then fall back. They can't escape the package.
  • Windows: With "exports": {"./shim": "./dist/shim.js"} and a nested shim/package.json, require('pkg\\shim') in Node gets the nested package's main, ignoring "exports". With the flag on, Metro normalises the backslash, so it gets dist/shim.js. I've only confirmed Metro's side of this one from a review run and the code.
  • POSIX backslashes: With "exports": {"./f": "./lib\\f.js"}, Node resolves lib/f.js on any platform. Metro on macOS and Linux treats the backslash literally, so it warns about a missing file. Windows matches Node.
  • Windows trailing slashes: With a target like "./a.js/" or "./img.png/", Node 24 on Windows resolves the file. Metro warns and fails on every platform, matching Node on POSIX.
  • Pattern matches with the flag off: With "exports": {"./p/*": "./lib/*"} and the flag off, pkg/p/g/../f.js and pkg/p/g/node_modules/h.js resolve where Node throws ERR_INVALID_MODULE_SPECIFIER. Enabling resolver.unstable_resolveExportsFromPackageRoot fixes this.

Fixed

  • #2089: Fallback arrays in "exports" and "imports" are tried in order, including [{"never": "./dist/y.never.js"}, "./dist/y.js"], ["not-valid", "./dist/x.js"] and [null, "./dist/n.js"].
  • #2089: Targets with an empty segment, such as "./dist//c.js", resolve.
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