test(eslint-config): add tests for no-direct-currency-formatting rule

jest.test.config.js

@@ -1,29 +1,65 @@
 process.env.ENABLE_NEW_JSX_TRANSFORM = 'true';
 
 /**
- * @type {import('@jest/types').Config.ProjectConfig}
+ * This config uses Jest "projects" to run two test suites in a single
+ * `pnpm test` invocation, each with its own environment:
+ *
+ *  - "test"         — the main suite (jsdom). Uses the MC app preset which
+ *                     sets up window.app, localStorage mocks, etc.
+ *  - "eslint-rules" — custom ESLint rule tests (node). These use ESLint's
+ *                     RuleTester which requires `structuredClone` (available
+ *                     in Node but not in jsdom) and has no DOM dependencies.
+ *                     The MC app preset's setup files also assume jsdom
+ *                     (they write to `global.window`), so these tests cannot
+ *                     run under the main project.
+ *
+ * If you add more custom ESLint rules under
+ * packages/eslint-config-mc-app/rules/, their *.spec.js files will be
+ * picked up automatically by the "eslint-rules" project.
+ *
+ * @type {import('@jest/types').Config.InitialOptions}
  */
 
 module.exports = {
-  preset: '@commercetools-frontend/jest-preset-mc-app/typescript',
-  moduleDirectories: [
-    'application-templates/',
-    'custom-views-templates/',
-    'packages/',
-    'playground/',
-    'node_modules/',
+  projects: [
+    // Main application test suite — jsdom environment.
+    {
+      displayName: 'test',
+      preset: '@commercetools-frontend/jest-preset-mc-app/typescript',
+      moduleDirectories: [
+        'application-templates/',
+        'custom-views-templates/',
+        'packages/',
+        'playground/',
+        'node_modules/',
+      ],
+      modulePathIgnorePatterns: [
+        '.cache',
+        'build',
+        'dist',
+        'public/',
+        'examples',
+        'packages-backend/',
+      ],
+      testPathIgnorePatterns: [
+        '/node_modules/',
+        // Excluded here because these tests need the node environment (see below).
+        'packages/eslint-config-mc-app/rules/',
+      ],
+      transformIgnorePatterns: [
+        // Transpile also our local packages as they are only symlinked.
+        'node_modules/(?!(@commercetools-[frontend|backend]+)/)',
+      ],
+      testEnvironment: 'jsdom',
+    },
+    // Custom ESLint rule tests — node environment, no preset/setup files.
+    // `transform: {}` disables Babel so the MC app preset's babel-plugin-istanbul
+    // doesn't conflict with Jest's own coverage instrumentation.
+    {
+      displayName: 'eslint-rules',
+      testEnvironment: 'node',
+      testMatch: ['<rootDir>/packages/eslint-config-mc-app/rules/**/*.spec.js'],
+      transform: {},
+    },
   ],
-  modulePathIgnorePatterns: [
-    '.cache',
-    'build',
-    'dist',
-    'public/',
-    'examples',
-    'packages-backend/',
-  ],
-  transformIgnorePatterns: [
-    // Transpile also our local packages as they are only symlinked.
-    'node_modules/(?!(@commercetools-[frontend|backend]+)/)',
-  ],
-  testEnvironment: 'jsdom',
 };