fix(@jest/resolve): replace unmatched capture group with empty string…

… instead of "undefined" (#14507)

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 ### Fixes
 
+- `[jest-resolver]` Replace unmatched capture groups in `moduleNameMapper` with empty string instead of `undefined` ([#14507](https://github.com/jestjs/jest/pull/14507))
 - `[jest-snapshot]` Allow for strings as well as template literals in inline snapshots ([#14465](https://github.com/jestjs/jest/pull/14465))
 - `[@jest/test-sequencer]` Calculate test runtime if `perStats.duration` is missing ([#14473](https://github.com/jestjs/jest/pull/14473))
 

packages/jest-resolve/src/__tests__/resolve.test.ts

@@ -561,6 +561,25 @@ describe('resolveModule', () => {
     expect(mockUserResolver).toHaveBeenCalled();
     expect(mockUserResolver.mock.calls[0][0]).toBe('fs');
   });
+
+  it('handles unmatched capture groups correctly', () => {
+    const resolver = new Resolver(moduleMap, {
+      extensions: ['.js'],
+      moduleNameMapper: [
+        {
+          moduleName: './__mocks__/foo$1',
+          regex: /^@Foo(\/.*)?$/,
+        },
+      ],
+    } as ResolverConfig);
+    const src = require.resolve('../');
+    expect(resolver.resolveModule(src, '@Foo')).toBe(
+      require.resolve('../__mocks__/foo.js'),
+    );
+    expect(resolver.resolveModule(src, '@Foo/bar')).toBe(
+      require.resolve('../__mocks__/foo/bar/index.js'),
+    );
+  });
 });
 
 describe('resolveModuleAsync', () => {

packages/jest-resolve/src/resolver.ts

@@ -439,7 +439,7 @@ export default class Resolver {
       ? (moduleName: string) =>
           moduleName.replace(
             /\$([0-9]+)/g,
-            (_, index) => matches[parseInt(index, 10)],
+            (_, index) => matches[parseInt(index, 10)] || '',
           )
       : (moduleName: string) => moduleName;
   }