Update getStorybookMain to throw an error if stories are not found in…

… main.js

src/util/__snapshots__/getStorybookMain.test.ts.snap

@@ -0,0 +1,9 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`getStorybookMain should throw an error if no configuration is found 1`] = `"Could not load main.js in .storybook. Is the config directory correct? You can change it by using --config-dir <path-to-dir>"`;
+
+exports[`getStorybookMain should throw an error if no stories are found 1`] = `
+"Could not find stories in main.js in .storybook. 
+If you are using a mono-repository, please run the test-runner only against your sub-package, which contains a .storybook folder with \\"stories\\" defined in main.js.
+You can change the config directory by using --config-dir <path-to-dir>"
+`;

src/util/getStorybookMain.test.ts

@@ -5,7 +5,17 @@ jest.mock('@storybook/core-common');
 
 describe('getStorybookMain', () => {
   it('should throw an error if no configuration is found', () => {
-    expect(() => getStorybookMain('.storybook')).toThrow();
+    expect(() => getStorybookMain('.storybook')).toThrowErrorMatchingSnapshot();
+  });
+
+  it('should throw an error if no stories are found', () => {
+    const mockedMain = {
+      stories: [],
+    };
+
+    jest.spyOn(coreCommon, 'serverRequire').mockImplementation(() => mockedMain);
+
+    expect(() => getStorybookMain('.storybook')).toThrowErrorMatchingSnapshot();
   });
 
   it('should return mainjs', () => {

src/util/getStorybookMain.ts

@@ -1,6 +1,7 @@
 import { join, resolve } from 'path';
 import { serverRequire } from '@storybook/core-common';
 import type { StorybookConfig } from '@storybook/types';
+import dedent from 'ts-dedent';
 
 let storybookMainConfig: StorybookConfig;
 
@@ -10,11 +11,22 @@ export const getStorybookMain = (configDir: string) => {
   }
 
   storybookMainConfig = serverRequire(join(resolve(configDir), 'main'));
+
   if (!storybookMainConfig) {
     throw new Error(
       `Could not load main.js in ${configDir}. Is the config directory correct? You can change it by using --config-dir <path-to-dir>`
     );
   }
 
+  if (!storybookMainConfig.stories || storybookMainConfig.stories.length === 0) {
+    throw new Error(
+      dedent`
+        Could not find stories in main.js in ${configDir}. 
+        If you are using a mono-repository, please run the test-runner only against your sub-package, which contains a .storybook folder with "stories" defined in main.js.
+        You can change the config directory by using --config-dir <path-to-dir>
+        `
+    );
+  }
+
   return storybookMainConfig;
 };