CLI: Avoid mixed CSF in files with unconventional stories

- Added early exit condition in `storyToCsfFactory` to skip transformation when no stories are detected, preventing mixed CSF syntax.
- Updated tests to verify the new behavior, ensuring proper logging when no stories are transformed.
- Mocked logger's log method for improved test coverage and validation of warning messages.

code/lib/cli-storybook/src/codemod/helpers/story-to-csf-factory.test.ts

@@ -10,6 +10,7 @@ import { storyToCsfFactory } from './story-to-csf-factory';
 
 vi.mock('storybook/internal/node-logger', () => ({
   logger: {
+    log: vi.fn(),
     warn: vi.fn(),
   },
 }));
@@ -34,25 +35,30 @@ describe('stories codemod', () => {
         transform(dedent`
             const meta = { title: 'Component' };
             export default meta;
+            export const A = {};
           `)
       ).resolves.toMatchInlineSnapshot(`
         import preview from '#.storybook/preview';
 
         const meta = preview.meta({ title: 'Component' });
+        export const A = meta.story();
       `);
     });
 
     it('should transform and wrap inline default exported meta', async () => {
       await expect(
         transform(dedent`
             export default { title: 'Component' };
+            export const A = {};
           `)
       ).resolves.toMatchInlineSnapshot(`
         import preview from '#.storybook/preview';
 
         const meta = preview.meta({
           title: 'Component',
         });
+
+        export const A = meta.story();
       `);
     });
 
@@ -61,11 +67,13 @@ describe('stories codemod', () => {
         transform(dedent`
             const componentMeta = { title: 'Component' };
             export default componentMeta;
+            export const A = {};
           `)
       ).resolves.toMatchInlineSnapshot(`
         import preview from '#.storybook/preview';
 
         const componentMeta = preview.meta({ title: 'Component' });
+        export const A = componentMeta.story();
       `);
     });
 
@@ -349,6 +357,7 @@ describe('stories codemod', () => {
                 source: dedent`
                   import preview, { extra } from '../../../.storybook/preview';
                   export default {};
+                  export const A = {};
                 `,
                 path: 'Component.stories.tsx',
               },
@@ -359,6 +368,7 @@ describe('stories codemod', () => {
           import preview, { extra } from '#.storybook/preview';
 
           const meta = preview.meta({});
+          export const A = meta.story();
         `);
 
         await expect(
@@ -369,6 +379,7 @@ describe('stories codemod', () => {
                 source: dedent`
                   import preview, { extra } from '#.storybook/preview';
                   export default {};
+                  export const A = {};
                 `,
                 path: 'Component.stories.tsx',
               },
@@ -379,6 +390,7 @@ describe('stories codemod', () => {
           import preview, { extra } from '../../preview';
 
           const meta = preview.meta({});
+          export const A = meta.story();
         `);
       } finally {
         relativeMock.mockRestore();
@@ -666,6 +678,24 @@ describe('stories codemod', () => {
       // expect(transformed).toContain('C = meta.story');
     });
 
+    it('should bail transformation when no stories can be transformed', async () => {
+      const source = dedent`
+        export default {
+          title: 'Component',
+        };
+      `;
+      const transformed = await transform(source);
+      const formattedSource = await formatFileContent('Component.stories.tsx', source);
+      expect(transformed).toEqual(formattedSource);
+
+      expect(transformed).not.toContain('preview.meta');
+      expect(transformed).not.toContain('meta.story');
+
+      expect(vi.mocked(logger.warn).mock.calls[0][0]).toMatchInlineSnapshot(
+        `Skipping codemod for Component.stories.tsx: no stories were transformed. Either there are no stories, file has been already transformed or some stories are written in an unsupported format.`
+      );
+    });
+
     it('should bail transformation and warn if some stories are not transformed to avoid mixed CSF formats', async () => {
       const source = dedent`
         export default {
@@ -700,7 +730,7 @@ describe('stories codemod', () => {
       const formattedSource = await formatFileContent('Component.stories.tsx', source);
       expect(transformed).toEqual(formattedSource);
 
-      expect(logger.warn).not.toHaveBeenCalled();
+      expect(logger.log).not.toHaveBeenCalled();
     });
   });
 });

code/lib/cli-storybook/src/codemod/helpers/story-to-csf-factory.ts

@@ -239,12 +239,16 @@ export async function storyToCsfFactory(
     },
   });
 
+  // If no stories were transformed, bail early to avoid having a mixed CSF syntax and therefore a broken indexer.
+  if (transformedStoryExports.size === 0) {
+    logger.warn(
+      `Skipping codemod for ${info.path}: no stories were transformed. Either there are no stories, file has been already transformed or some stories are written in an unsupported format.`
+    );
+    return info.source;
+  }
+
   // If some stories were detected but not all could be transformed, we skip the codemod to avoid mixed csf syntax and therefore a broken indexer.
-  if (
-    detectedStoryNames.length > 0 &&
-    transformedStoryExports.size > 0 &&
-    transformedStoryExports.size !== detectedStoryNames.length
-  ) {
+  if (detectedStoryNames.length > 0 && transformedStoryExports.size !== detectedStoryNames.length) {
     logger.warn(
       `Skipping codemod for ${info.path}:\nSome of the detected stories [${detectedStoryNames
         .map((name) => `"${name}"`)