FEC-753: Add shared no-direct-currency formatting rule

.changeset/fec-738-no-direct-currency-formatting.md

@@ -0,0 +1,43 @@
+---
+'@commercetools-frontend/eslint-config-mc-app': minor
+---
+
+Add bundled `no-direct-currency-formatting` rule via the `@commercetools-frontend/eslint-config-mc-app/rules` inline plugin.
+
+This rule disallows direct currency formatting through `intl.formatNumber`, `intl.formatCurrency`, `new Intl.NumberFormat` when using a `currency` option or `style: 'currency'`, and `<FormattedNumber />` from `react-intl`.
+
+Use a shared currency formatting wrapper instead, and allowlist that wrapper path if needed.
+
+## How to update
+
+Enable the bundled rule in your project config:
+
+```js
+// eslint.config.js
+import mcAppConfig from '@commercetools-frontend/eslint-config-mc-app';
+
+export default [
+  ...mcAppConfig,
+  {
+    files: ['**/*.{js,jsx,ts,tsx}'],
+    rules: {
+      '@commercetools-frontend/eslint-config-mc-app/rules/no-direct-currency-formatting':
+        [
+          'error',
+          {
+            allowedWrapperPaths: [
+              'src/utils/money.js', // path to your shared wrapper implementation
+            ],
+          },
+        ],
+    },
+  },
+];
+```
+
+If you need to customize the wrapper allowlist, pass `allowedWrapperPaths` as shown above.
+
+## Why
+
+Direct currency formatting is hard to standardize across applications and can drift in behavior over time.
+Enforcing a shared wrapper keeps formatting logic consistent, testable, and centrally maintainable.

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',
 };

packages/eslint-config-mc-app/index.js

@@ -90,6 +90,9 @@ const { statusCode, allSupportedExtensions } = require('./helpers/eslint');
 const hasJsxRuntime = require('./helpers/has-jsx-runtime');
 const { craRules } = require('./helpers/rules-presets');
 
+// Bundled custom rules
+const noDirectCurrencyFormattingRule = require('./rules/no-direct-currency-formatting');
+
 /**
  * ESLint flat config format for @commercetools-frontend/eslint-config-mc-app
  * @type {import("eslint").Linter.FlatConfig[]}
@@ -124,6 +127,11 @@ module.exports = [
       'jsx-a11y': jsxA11yPlugin,
       prettier: prettierPlugin,
       cypress: cypressPlugin,
+      '@commercetools-frontend/eslint-config-mc-app/rules': {
+        rules: {
+          'no-direct-currency-formatting': noDirectCurrencyFormattingRule,
+        },
+      },
     },
     settings: {
       'import/resolver': {