Add ESLint flat config with CI enforcement

.github/workflows/test.yml

@@ -15,9 +15,30 @@ concurrency:
   cancel-in-progress: true
 
 env:
-  NODE_VERSION: 18
+  NODE_VERSION: 22
 
 jobs:
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          submodules: recursive
+
+      - name: Set up Node.js ${{ env.NODE_VERSION }}
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
+        with:
+          cache: yarn
+          node-version: ${{ env.NODE_VERSION }}
+
+      - name: Install dependencies
+        run: script/bootstrap
+
+      - name: Lint
+        run: yarn lint
+
   build_frontend:
     name: Test build
     runs-on: ubuntu-latest

.yarnrc.yml

@@ -7,4 +7,4 @@ enableGlobalCache: false
 
 nodeLinker: node-modules
 
-yarnPath: ./homeassistant-frontend/.yarn/releases/yarn-4.5.3.cjs
+yarnPath: ./homeassistant-frontend/.yarn/releases/yarn-4.12.0.cjs

eslint.config.mjs

@@ -0,0 +1,57 @@
+import eslint from "@eslint/js";
+import tseslint from "typescript-eslint";
+import eslintConfigPrettier from "eslint-config-prettier";
+import pluginLit from "eslint-plugin-lit";
+import pluginWc from "eslint-plugin-wc";
+import pluginLitA11y from "eslint-plugin-lit-a11y";
+import globals from "globals";
+
+export default tseslint.config(
+  {
+    ignores: [
+      "homeassistant-frontend/**",
+      "hacs_frontend/**",
+      "build/**",
+      "node_modules/**",
+      "script/**",
+    ],
+  },
+  eslint.configs.recommended,
+  ...tseslint.configs.recommended,
+  pluginLit.configs["flat/recommended"],
+  pluginWc.configs["flat/recommended"],
+  pluginLitA11y.configs.recommended,
+  eslintConfigPrettier,
+  {
+    languageOptions: {
+      ecmaVersion: "latest",
+      sourceType: "module",
+      globals: {
+        ...globals.browser,
+        __DEV__: "readonly",
+        __DEMO__: "readonly",
+        __BUILD__: "readonly",
+        __VERSION__: "readonly",
+        __STATIC_PATH__: "readonly",
+        __SUPERVISOR__: "readonly",
+      },
+    },
+    rules: {
+      "no-bitwise": "error",
+      "no-console": "error",
+      "no-restricted-globals": ["error", "event"],
+      "no-restricted-syntax": ["error", "LabeledStatement", "WithStatement"],
+      "no-shadow": "off",
+      "@typescript-eslint/no-shadow": "error",
+      "@typescript-eslint/ban-ts-comment": "off",
+      "@typescript-eslint/no-use-before-define": "off",
+      "@typescript-eslint/no-non-null-assertion": "off",
+      "@typescript-eslint/no-explicit-any": "off",
+      "@typescript-eslint/no-unused-vars": "off",
+      "@typescript-eslint/explicit-function-return-type": "off",
+      "@typescript-eslint/explicit-module-boundary-types": "off",
+      "wc/no-self-class": "off",
+      "lit/attribute-value-entities": "off",
+    },
+  },
+);