build(deps-dev): migrate to ESLint 10 and flat config

.github/dependabot.yml

@@ -20,16 +20,12 @@ updates:
               patterns: ['@wordpress/*']
               dependency-type: 'production'
       ignore:
-          - dependency-name: 'eslint' # eslint@>=9.x.x blocked by https://github.com/WordPress/gutenberg/issues/64782
-            update-types: ['version-update:semver-major']
           - dependency-name: 'react' # react@>=19.x.x blocked by https://github.com/WordPress/gutenberg/issues/71336
             update-types: ['version-update:semver-major']
           - dependency-name: 'react-dom' # react@>=19.x.x blocked by https://github.com/WordPress/gutenberg/issues/71336
             update-types: ['version-update:semver-major']
           - dependency-name: 'jquery' # jquery@>=4.x.x blocked by https://core.trac.wordpress.org/ticket/60478
             update-types: ['version-update:semver-major']
-          - dependency-name: 'eslint-plugin-react-refresh' # eslint-plugin-react-refresh@>=0.5.x requires eslint@>=9.x.x, blocked by https://github.com/WordPress/gutenberg/issues/64782
-            update-types: ['version-update:semver-minor']
 
     - package-ecosystem: 'bundler'
       directory: '/'

AGENTS.md

@@ -39,7 +39,7 @@ See `docs/code/local-wordpress.md` for detailed setup instructions and troublesh
 
 The project follows WordPress coding standards for JavaScript:
 
--   **ESLint**: Uses `@wordpress/eslint-plugin/recommended` configuration
+-   **ESLint**: Flat config in `eslint.config.js`, extending `@wordpress/eslint-plugin`'s `recommended` config. Its `test-playwright` config applies to `e2e/` except the `*.test.js` files, which are vitest unit tests rather than Playwright specs.
 -   **Prettier**: Uses `@wordpress/prettier-config` for code formatting
 
 ### Function Ordering Convention

bin/prep-translations.js

@@ -323,7 +323,7 @@ function hasValidTranslations( locale ) {
 		const content = fs.readFileSync( filePath, 'utf8' );
 		const translations = JSON.parse( content );
 		return translations && typeof translations === 'object';
-	} catch ( err ) {
+	} catch {
 		return false;
 	}
 }

eslint.config.js

@@ -0,0 +1,81 @@
+/**
+ * External dependencies
+ */
+import { defineConfig, globalIgnores } from 'eslint/config';
+import wordpress from '@wordpress/eslint-plugin';
+import reactRefresh from 'eslint-plugin-react-refresh';
+import globals from 'globals';
+
+export default defineConfig( [
+	globalIgnores( [
+		// Flat config, unlike eslintrc, does not ignore dot directories, so the
+		// SwiftPM build directory has to be named explicitly.
+		'.build/',
+		'android/',
+		'build/',
+		'dist/',
+		'e2e/test-results/',
+		'ios/',
+		'playwright-report/',
+		'vendor/',
+	] ),
+	{
+		files: [ '**/*.{js,jsx,cjs,mjs}' ],
+		extends: [ wordpress.configs.recommended ],
+		languageOptions: {
+			globals: globals.browser,
+		},
+		settings: {
+			'import/resolver': {
+				// The WordPress config registers only the TypeScript resolver,
+				// which honors `exports` and so resolves neither the
+				// `@wordpress/block-editor` subpaths added in `patches/`, whose
+				// targets omit the `.mjs` extension, nor
+				// `@wordpress/preferences-persistence`, whose `types` condition
+				// names build output it does not publish. The node resolver
+				// ignores `exports` and resolves both.
+				node: {
+					extensions: [ '.js', '.jsx', '.mjs', '.cjs' ],
+				},
+			},
+		},
+		plugins: {
+			'react-refresh': reactRefresh,
+		},
+		rules: {
+			'react-refresh/only-export-components': [
+				'warn',
+				{ allowConstantExport: true },
+			],
+		},
+	},
+	{
+		files: [ 'e2e/**/*.js' ],
+		// Unit tests sit alongside the specs but run under vitest, whose `it()`
+		// blocks the Playwright rules do not recognize as tests.
+		ignores: [ 'e2e/**/*.test.js' ],
+		extends: [ wordpress.configs[ 'test-playwright' ] ],
+		rules: {
+			// Discourage deprecated Playwright APIs in favor of locators, aligned
+			// with the upstream Gutenberg ESLint configuration.
+			'no-restricted-syntax': [
+				'error',
+				{
+					selector: 'CallExpression[callee.property.name="$"]',
+					message:
+						'`$` is discouraged, please use `locator` instead.',
+				},
+				{
+					selector: 'CallExpression[callee.property.name="$$"]',
+					message:
+						'`$$` is discouraged, please use `locator` instead.',
+				},
+				{
+					selector:
+						'CallExpression[callee.object.name="page"][callee.property.name="waitForTimeout"]',
+					message: 'Prefer page.locator instead.',
+				},
+			],
+		},
+	},
+] );