CSF: Warn when there are no exported stories

lib/core/src/client/preview/loadCsf.test.ts

@@ -1,8 +1,13 @@
 import { ConfigApi, ClientApi, StoryStore } from '@storybook/client-api';
+import { logger } from '@storybook/client-logger';
 import { RequireContext } from './types';
 
 import { loadCsf } from './loadCsf';
 
+jest.mock('@storybook/client-logger', () => ({
+  logger: { warn: jest.fn() },
+}));
+
 let cbs: ((data: any) => void)[];
 let mod: NodeModule;
 beforeEach(() => {
@@ -365,4 +370,34 @@ describe('core.preview.loadCsf', () => {
     expect(storyStore.incrementRevision).toHaveBeenCalled();
     expect(mockedStoriesOf).toHaveBeenCalledWith('a', true);
   });
+
+  it('gives a warning if there are no exported stories', () => {
+    const { configure } = makeMocks();
+
+    const input = {
+      a: {
+        default: {
+          title: 'MissingExportsComponent',
+        },
+        // no named exports, will not present a story
+      },
+    };
+    configure(makeRequireContext(input), mod, 'react');
+    expect(logger.warn).toHaveBeenCalled();
+  });
+
+  it('does not give a warning if there are exported stories', () => {
+    const { configure } = makeMocks();
+
+    const input = {
+      a: {
+        default: {
+          title: 'MissingExportsComponent',
+        },
+        x: () => 0,
+      },
+    };
+    configure(makeRequireContext(input), mod, 'react');
+    expect(logger.warn).not.toHaveBeenCalled();
+  });
 });

lib/core/src/client/preview/loadCsf.ts

@@ -1,6 +1,7 @@
 import { ConfigApi, ClientApi, StoryStore } from '@storybook/client-api';
 import { isExportStory, storyNameFromExport, toId } from '@storybook/csf';
 import { logger } from '@storybook/client-logger';
+import dedent from 'ts-dedent';
 
 import { Loadable, LoaderFunction, RequireContext } from './types';
 
@@ -113,7 +114,16 @@ const loadStories = (
       kind.addDecorator(decorator);
     });
 
-    Object.keys(exports).forEach((key) => {
+    const storyExports = Object.keys(exports);
+    if (storyExports.length === 0) {
+      logger.warn(
+        dedent`Found a story file for "${kindName}" but no exported stories.
+        Check the docs for reference: https://storybook.js.org/docs/formats/component-story-format/`
+      );
+      return;
+    }
+
+    storyExports.forEach((key) => {
       if (isExportStory(key, meta)) {
         const storyFn = exports[key];
         const { name, parameters, decorators, args, argTypes } = storyFn.story || {};