refactor(linting): migrate eslint from v8 to v9 with flat config

eslint.config.js

@@ -0,0 +1,185 @@
+import eslint from '@eslint/js'
+import eslintConfigPrettier from 'eslint-config-prettier'
+import eslintPluginImportX from 'eslint-plugin-import-x'
+import eslintPluginLit from 'eslint-plugin-lit'
+import eslintPluginWc from 'eslint-plugin-wc'
+import eslintPluginLitA11y from 'eslint-plugin-lit-a11y'
+import eslintPluginN from 'eslint-plugin-n'
+import eslintPluginPromise from 'eslint-plugin-promise'
+import eslintPluginNoOnlyTests from 'eslint-plugin-no-only-tests'
+import globals from 'globals'
+
+const productionFiles = ['src/**/*.js', 'src/**/*.ts']
+const testFiles = ['test/**/*.js']
+
+const config = [
+  // Ignore build artifacts and dependencies
+  {
+    ignores: [
+      'dist/**',
+      'coverage/**',
+      'node_modules/**',
+      '*.min.js',
+      '**/*.min.js',
+    ],
+  },
+  // Rollup config file (uses Node.js globals)
+  {
+    files: ['rollup.config.js'],
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+      ecmaVersion: 2023,
+      sourceType: 'module',
+    },
+  },
+  // Main production source files
+  {
+    files: productionFiles,
+    plugins: {
+      'import-x': eslintPluginImportX,
+      lit: eslintPluginLit,
+      wc: eslintPluginWc,
+      'lit-a11y': eslintPluginLitA11y,
+      n: eslintPluginN,
+      promise: eslintPluginPromise,
+      'no-only-tests': eslintPluginNoOnlyTests,
+    },
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        ...globals.es2021,
+      },
+      ecmaVersion: 2023,
+      sourceType: 'module',
+    },
+    rules: {
+      ...eslintPluginLit.configs.recommended.rules,
+      ...eslintPluginWc.configs.recommended.rules,
+      ...eslintPluginLitA11y.configs.recommended.rules,
+      'no-only-tests/no-only-tests': 'error',
+    },
+  },
+
+  // Import rules from import-x
+  {
+    files: productionFiles,
+    ...eslintPluginImportX.configs['flat/recommended'],
+  },
+
+  // Node.js rules from eslint-plugin-n
+  {
+    files: productionFiles,
+    plugins: {
+      n: eslintPluginN,
+    },
+    rules: {
+      'n/handle-callback-err': ['error', '^(err|error)$'],
+      'n/no-callback-literal': 'error',
+      'n/no-deprecated-api': 'error',
+      'n/no-exports-assign': 'error',
+      'n/no-new-require': 'error',
+      'n/no-path-concat': 'error',
+      'n/process-exit-as-throw': 'error',
+    },
+  },
+
+  // Promise rules
+  {
+    files: productionFiles,
+    plugins: {
+      promise: eslintPluginPromise,
+    },
+    rules: {
+      'promise/param-names': 'error',
+    },
+  },
+
+  // Test files
+  {
+    files: testFiles,
+    plugins: {
+      'import-x': eslintPluginImportX,
+      'no-only-tests': eslintPluginNoOnlyTests,
+    },
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        ...globals.es2021,
+        ...globals.node,
+      },
+      ecmaVersion: 2023,
+      sourceType: 'module',
+    },
+    rules: {
+      'import-x/no-extraneous-dependencies': ['error', {devDependencies: true}],
+      'no-unused-expressions': 'off',
+      'no-only-tests/no-only-tests': 'error',
+    },
+  },
+
+  // Vitest config file
+  {
+    files: ['vitest.config.js'],
+    plugins: {
+      'import-x': eslintPluginImportX,
+    },
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+      ecmaVersion: 2023,
+      sourceType: 'module',
+    },
+    rules: {
+      'import-x/no-unresolved': 'off',
+    },
+  },
+
+  // TypeScript files
+  {
+    files: ['**/*.ts'],
+    plugins: {
+      'import-x': eslintPluginImportX,
+    },
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        ...globals.es2021,
+      },
+      ecmaVersion: 2023,
+      sourceType: 'module',
+    },
+    rules: {
+      'no-undef': 'off',
+    },
+  },
+
+  // ESLint recommended rules (must be last to allow overrides)
+  eslint.configs.recommended,
+
+  // no-unused-vars: allow unused variables in destructuring with rest operator
+  // This mirrors the behavior of eslint-config-standard which did not flag
+  // patterns like {extended, profile, ...rest} where the individual keys are
+  // intentionally discarded and only the rest is used.
+  {
+    files: productionFiles,
+    rules: {
+      'no-unused-vars': [
+        'error',
+        {
+          argsIgnorePattern: '^_',
+          varsIgnorePattern: '^_',
+          caughtErrorsIgnorePattern: '^_',
+          ignoreRestSiblings: true,
+        },
+      ],
+    },
+  },
+
+  // Prettier must be last to disable conflicting rules
+  eslintConfigPrettier,
+]
+
+export default config