fix(js): handle outputFileName correctly when generating package json (#28915)

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Fixes #28894

packages/js/src/utils/package-json/update-package-json.spec.ts

@@ -405,6 +405,38 @@ describe('getUpdatedPackageJsonContent', () => {
       },
     });
   });
+
+  it('should handle outputFileName correctly', () => {
+    expect(
+      getUpdatedPackageJsonContent(
+        {
+          name: 'test',
+          version: '0.0.1',
+        },
+        {
+          main: 'proj/src/index.ts',
+          outputPath: 'dist/proj',
+          projectRoot: 'proj',
+          format: ['cjs'],
+          generateExportsField: true,
+          outputFileName: 'src/index.js',
+        }
+      )
+    ).toEqual({
+      name: 'test',
+      main: './src/index.js',
+      types: './src/index.d.ts',
+      version: '0.0.1',
+      type: 'commonjs',
+      exports: {
+        '.': {
+          default: './src/index.js',
+          types: './src/index.d.ts',
+        },
+        './package.json': './package.json',
+      },
+    });
+  });
 });
 
 describe('updatePackageJson', () => {

packages/js/src/utils/package-json/update-package-json.ts

@@ -242,7 +242,7 @@ export function getExports(
 ): Exports {
   const outputDir = getOutputDir(options);
   const mainFile = options.outputFileName
-    ? options.outputFileName.replace(/\.[tj]s$/, '')
+    ? basename(options.outputFileName).replace(/\.[tj]s$/, '')
     : basename(options.main).replace(/\.[tj]s$/, '');
   const exports: Exports = {
     '.': outputDir + mainFile + options.fileExt,
@@ -377,9 +377,9 @@ export function getOutputDir(
     : options.outputPath;
   const relativeOutputPath = relative(packageJsonDir, options.outputPath);
   const relativeMainDir = options.outputFileName
-    ? ''
+    ? dirname(options.outputFileName)
     : relative(options.rootDir ?? options.projectRoot, dirname(options.main));
-  const outputDir = join(relativeOutputPath, relativeMainDir);
+  const outputDir = joinPathFragments(relativeOutputPath, relativeMainDir);
 
   return outputDir === '.' ? `./` : `./${outputDir}/`;
 }