Resolver: Fix browser-spec bare-specifier->relative path redirect for nested/first-party package.jsons

packages/metro-resolver/src/__tests__/browser-spec-test.js

@@ -99,4 +99,61 @@ describe('browser field spec', () => {
       });
     });
   });
+
+  describe('replace specific files', () => {
+    test('should resolve a bare-specifier redirect relative to the origin package root, not its containing directory', () => {
+      // Per the browser spec, paths in the `browser` map are relative to the
+      // package.json file location. When the origin module lives in a
+      // subdirectory of its package (here `lib/nested/`), the redirect target
+      // must still resolve against the package root.
+      const packageJson = {
+        name: 'origin-pkg',
+        main: 'lib/nested/index.js',
+        browser: {
+          'foo-pkg': './shims/foo.js',
+        },
+      };
+      const context = {
+        ...createResolutionContext({
+          '/root/node_modules/origin-pkg/package.json':
+            JSON.stringify(packageJson),
+          '/root/node_modules/origin-pkg/lib/nested/index.js': '',
+          '/root/node_modules/origin-pkg/shims/foo.js': '',
+        }),
+        originModulePath: '/root/node_modules/origin-pkg/lib/nested/index.js',
+        mainFields: ['browser', 'main'],
+      };
+
+      expect(Resolver.resolve(context, 'foo-pkg', null)).toEqual({
+        type: 'sourceFile',
+        filePath: '/root/node_modules/origin-pkg/shims/foo.js',
+      });
+    });
+
+    test('should resolve a bare-specifier redirect for an origin outside of `node_modules`', () => {
+      // Project-level package.json — there is no enclosing `node_modules`
+      // segment, so the old heuristic of slicing after `node_modules/` would
+      // misbehave. The redirect must resolve against the package root.
+      const context = {
+        ...createResolutionContext({
+          '/root/project/package.json': JSON.stringify({
+            name: 'project',
+            main: 'src/index.js',
+            browser: {
+              'foo-pkg': './shims/foo.js',
+            },
+          }),
+          '/root/project/src/index.js': '',
+          '/root/project/shims/foo.js': '',
+        }),
+        originModulePath: '/root/project/src/index.js',
+        mainFields: ['browser', 'main'],
+      };
+
+      expect(Resolver.resolve(context, 'foo-pkg', null)).toEqual({
+        type: 'sourceFile',
+        filePath: '/root/project/shims/foo.js',
+      });
+    });
+  });
 });

packages/metro-resolver/src/resolve.js

@@ -135,22 +135,16 @@ export default function resolve(
   // If the specifier was redirected to a relative path
   if (
     maybeRedirectedSpecifier != null &&
+    closestPackageToOrigin != null && // Implied by maybeRedirectedSpecifier != null
     isRelativeImport(maybeRedirectedSpecifier)
   ) {
-    // TODO: (robhogan) This isn't right - per browser spec: "All paths for
-    // browser fields are relative to the package.json file location". The
-    // *closest* package.json is the relevant one for browser spec, regardless
-    // of the closest node_modules.
-
-    // derive absolute path /.../node_modules/originModuleDir/specifier
-    const fromModuleParentIdx =
-      originModulePath.lastIndexOf('node_modules' + path.sep) + 13;
-    const originModuleDir = originModulePath.slice(
-      0,
-      originModulePath.indexOf(path.sep, fromModuleParentIdx),
+    // Per the "browser" spec: "All paths for browser fields are relative to
+    // the package.json file location". `closestPackageToOrigin` is the package
+    // that provided the redirect, so join relative paths to its `rootPath`.
+    const absPath = path.resolve(
+      closestPackageToOrigin.rootPath,
+      maybeRedirectedSpecifier,
     );
-
-    const absPath = path.join(originModuleDir, maybeRedirectedSpecifier);
     const result = resolveModulePath(context, absPath, platform);
     if (result.type === 'failed') {
       throw new FailedToResolvePathError(result.candidates);