build!: upgrade to ESLint 9

eslint.config.js

@@ -0,0 +1,18 @@
+import globals from "globals";
+import eslintJs from "@eslint/js";
+
+export default [
+	{ ignores: ["test/fixtures/**/*", "test/__snapshots__/**/*"] },
+
+	eslintJs.configs.recommended,
+
+	{
+		languageOptions: {
+			globals: { ...globals.node, ...globals.jest },
+			ecmaVersion: 2020,
+			sourceType: "module",
+		},
+
+		rules: { "no-empty": 0 },
+	},
+];

index.js

@@ -1,161 +1,199 @@
-export default {
-	// TODO: this is really only required for class property initializer methods, which are seeing declining usage.
-	// At some point, we should un-ship the custom parser and let ESLint use esprima.
-	parser: require.resolve('@babel/eslint-parser'),
-
-	// Currently ignored due to the custom parser.
-	parserOptions: {
-		ecmaVersion: 2020,
-		sourceType: 'module',
-		ecmaFeatures: {
-			modules: true,
-			impliedStrict: true,
-			jsx: true
-		},
-		requireConfigFile: false,
-		babelOptions: {
-			plugins: [
-				'@babel/plugin-syntax-class-properties',
-				'@babel/plugin-syntax-jsx'
-			]
-		}
-	},
+const babelEslintParser = require("@babel/eslint-parser");
+const eslintJs = require("@eslint/js");
+const compatPlugin = require("eslint-plugin-compat");
+const reactPlugin = require("eslint-plugin-react");
+const reactHooksPlugin = require("eslint-plugin-react-hooks");
+const globals = require("globals");
 
-	// We don't use plugin:react/recommended here to avoid React-specific rules.
-	extends: [
-		'eslint:recommended'
-	],
+/**
+ * @typedef {import('eslint').Linter.LegacyConfig} LegacyConfig
+ * @typedef {import('eslint').Linter.Config} Config
+ */
 
-	plugins: [
-		'compat',
-		'react',
-		'react-hooks'
-	],
+/** @type {(Config['languageOptions'] & LegacyConfig)['globals']} */
+const sharedGlobals = {
+	browser: true,
+	expect: true,
+	global: true,
+};
 
-	env: {
-		browser: true,
-		es6: true,
-		node: true
+/** @type {(Config & LegacyConfig)['parserOptions']} */
+const sharedParserOptions = {
+	ecmaVersion: 2020,
+	sourceType: "module",
+	ecmaFeatures: {
+		modules: true,
+		impliedStrict: true,
+		jsx: true,
 	},
-
-	globals: {
-		expect: true,
-		browser: true,
-		global: true
+	requireConfigFile: false,
+	babelOptions: {
+		plugins: [
+			"@babel/plugin-syntax-class-properties",
+			"@babel/plugin-syntax-jsx",
+		],
 	},
+};
 
-	settings: {
-		// Preact CLI provides these defaults
-		targets: ['last 2 versions'],
-		polyfills: ['fetch', 'Promise'],
-		react: {
-			// eslint-plugin-preact interprets this as "h.createElement",
-			// however we only care about marking h() as being a used variable.
-			pragma: 'h',
-			// We use "react 16.0" to avoid pushing folks to UNSAFE_ methods.
-			version: '16.0'
-		}
+/** @type {(Config & LegacyConfig)['settings']} */
+const sharedSettings = {
+	// Preact CLI provides these defaults
+	targets: ["last 2 versions"],
+	polyfills: ["fetch", "Promise"],
+	react: {
+		// eslint-plugin-preact interprets this as "h.createElement",
+		// however we only care about marking h() as being a used variable.
+		pragma: "h",
+		// We use "react 16.0" to avoid pushing folks to UNSAFE_ methods.
+		version: "16.0",
 	},
+};
 
-	rules: {
-		/**
-		 * Preact / JSX rules
-		 */
-		'react/no-deprecated': 2,
-		'react/react-in-jsx-scope': 0, // handled this automatically
-		'react/display-name': [1, { ignoreTranspilerName: false }],
-		'react/jsx-no-bind': [1, {
+/** @type {(Config & LegacyConfig)['rules']} */
+const sharedRules = {
+	/** Preact / JSX rules */
+	"react/no-deprecated": 2,
+	"react/react-in-jsx-scope": 0, // handled this automatically
+	"react/display-name": [1, { ignoreTranspilerName: false }],
+	"react/jsx-no-bind": [
+		1,
+		{
 			ignoreRefs: true,
 			allowFunctions: true,
-			allowArrowFunctions: true
-		}],
-		'react/jsx-no-comment-textnodes': 2,
-		'react/jsx-no-duplicate-props': 2,
-		'react/jsx-no-target-blank': 2,
-		'react/jsx-no-undef': 2,
-		'react/jsx-tag-spacing': [2, { beforeSelfClosing: 'always' }],
-		'react/jsx-uses-react': 2, // debatable
-		'react/jsx-uses-vars': 2,
-		'react/jsx-key': [2, { checkFragmentShorthand: true }],
-		'react/self-closing-comp': 2,
-		'react/prefer-es6-class': 2,
-		'react/prefer-stateless-function': 1,
-		'react/require-render-return': 2,
-		'react/no-danger': 1,
-		// Legacy APIs not supported in Preact:
-		'react/no-did-mount-set-state': 2,
-		'react/no-did-update-set-state': 2,
-		'react/no-find-dom-node': 2,
-		'react/no-is-mounted': 2,
-		'react/no-string-refs': 2,
+			allowArrowFunctions: true,
+		},
+	],
+	"react/jsx-no-comment-textnodes": 2,
+	"react/jsx-no-duplicate-props": 2,
+	"react/jsx-no-target-blank": 2,
+	"react/jsx-no-undef": 2,
+	"react/jsx-tag-spacing": [2, { beforeSelfClosing: "always" }],
+	"react/jsx-uses-react": 2, // debatable
+	"react/jsx-uses-vars": 2,
+	"react/jsx-key": [2, { checkFragmentShorthand: true }],
+	"react/self-closing-comp": 2,
+	"react/prefer-es6-class": 2,
+	"react/prefer-stateless-function": 1,
+	"react/require-render-return": 2,
+	"react/no-danger": 1,
+	// Legacy APIs not supported in Preact:
+	"react/no-did-mount-set-state": 2,
+	"react/no-did-update-set-state": 2,
+	"react/no-find-dom-node": 2,
+	"react/no-is-mounted": 2,
+	"react/no-string-refs": 2,
 
-		/**
-		 * Hooks
-		 */
-		'react-hooks/rules-of-hooks': 2,
-		'react-hooks/exhaustive-deps': 1,
+	/** Hooks */
+	"react-hooks/rules-of-hooks": 2,
+	"react-hooks/exhaustive-deps": 1,
 
-		/**
-		 * General JavaScript error avoidance
-		 */
-		'constructor-super': 2,
-		'no-caller': 2,
-		'no-const-assign': 2,
-		'no-delete-var': 2,
-		'no-dupe-class-members': 2,
-		'no-dupe-keys': 2,
-		'no-duplicate-imports': 2,
-		'no-else-return': 1,
-		'no-empty-pattern': 0,
-		'no-empty': 0,
-		'no-extra-parens': 0,
-		'no-iterator': 2,
-		'no-lonely-if': 2,
-		'no-mixed-spaces-and-tabs': [1, 'smart-tabs'],
-		'no-multi-str': 1,
-		'no-new-wrappers': 2,
-		'no-proto': 2,
-		'no-redeclare': 2,
-		'no-shadow-restricted-names': 2,
-		'no-shadow': 0,
-		'no-spaced-func': 2,
-		'no-this-before-super': 2,
-		'no-undef-init': 2,
-		'no-unneeded-ternary': 2,
-		'no-unused-vars': [2, {
-			args: 'after-used',
-			ignoreRestSiblings: true
-		}],
-		'no-useless-call': 1,
-		'no-useless-computed-key': 1,
-		'no-useless-concat': 1,
-		'no-useless-constructor': 1,
-		'no-useless-escape': 1,
-		'no-useless-rename': 1,
-		'no-var': 1,
-		'no-with': 2,
+	/** General JavaScript error avoidance */
+	"constructor-super": 2,
+	"no-caller": 2,
+	"no-const-assign": 2,
+	"no-delete-var": 2,
+	"no-dupe-class-members": 2,
+	"no-dupe-keys": 2,
+	"no-duplicate-imports": 2,
+	"no-else-return": 1,
+	"no-empty-pattern": 0,
+	"no-empty": 0,
+	"no-extra-parens": 0,
+	"no-iterator": 2,
+	"no-lonely-if": 2,
+	"no-mixed-spaces-and-tabs": [1, "smart-tabs"],
+	"no-multi-str": 1,
+	"no-new-wrappers": 2,
+	"no-proto": 2,
+	"no-redeclare": 2,
+	"no-shadow-restricted-names": 2,
+	"no-shadow": 0,
+	"no-spaced-func": 2,
+	"no-this-before-super": 2,
+	"no-undef-init": 2,
+	"no-unneeded-ternary": 2,
+	"no-unused-vars": [
+		2,
+		{
+			args: "after-used",
+			ignoreRestSiblings: true,
+		},
+	],
+	"no-useless-call": 1,
+	"no-useless-computed-key": 1,
+	"no-useless-concat": 1,
+	"no-useless-constructor": 1,
+	"no-useless-escape": 1,
+	"no-useless-rename": 1,
+	"no-var": 1,
+	"no-with": 2,
 
-		/**
-		 * General JavaScript stylistic rules (disabled)
-		 */
-		semi: 0,
-		strict: [2, 'never'], // assume type=module output (cli default)
-		'object-curly-spacing': [0, 'always'],
-		'rest-spread-spacing': 0,
-		'space-before-function-paren': [0, 'always'],
-		'space-in-parens': [0, 'never'],
-		'object-shorthand': 1,
-		'prefer-arrow-callback': 1,
-		'prefer-rest-params': 1,
-		'prefer-spread': 1,
-		'prefer-template': 1,
-		quotes: [0, 'single', {
+	/** General JavaScript stylistic rules (disabled) */
+	semi: 0,
+	strict: [2, "never"], // assume type=module output (cli default)
+	"object-curly-spacing": [0, "always"],
+	"rest-spread-spacing": 0,
+	"space-before-function-paren": [0, "always"],
+	"space-in-parens": [0, "never"],
+	"object-shorthand": 1,
+	"prefer-arrow-callback": 1,
+	"prefer-rest-params": 1,
+	"prefer-spread": 1,
+	"prefer-template": 1,
+	quotes: [
+		0,
+		"single",
+		{
 			avoidEscape: true,
-			allowTemplateLiterals: true
-		}],
-		'quote-props': [2, 'as-needed'],
-		radix: 1, // parseInt(x, 10)
-		'unicode-bom': 2,
-		'valid-jsdoc': 0
-	}
+			allowTemplateLiterals: true,
+		},
+	],
+	"quote-props": [2, "as-needed"],
+	radix: 1, // parseInt(x, 10)
+	"unicode-bom": 2,
+	"valid-jsdoc": 0,
 };
+
+/** @type {LegacyConfig} */
+const legacyConfig = {
+	// TODO: this is really only required for class property initializer methods, which are seeing declining usage.
+	// At some point, we should un-ship the custom parser and let ESLint use esprima.
+	parser: require.resolve("@babel/eslint-parser"),
+
+	// We don't use plugin:react/recommended here to avoid React-specific rules.
+	extends: ["eslint:recommended"],
+	plugins: ["compat", "react", "react-hooks"],
+	env: { browser: true, es6: true, node: true },
+
+	globals: { ...sharedGlobals },
+	parserOptions: { ...sharedParserOptions },
+	settings: { ...sharedSettings },
+	rules: { ...sharedRules },
+};
+
+/** @type {Config[]} */
+const flatConfig = [
+	eslintJs.configs.recommended,
+
+	{
+		plugins: {
+			compat: compatPlugin,
+			react: reactPlugin,
+			"react-hooks": reactHooksPlugin,
+		},
+		languageOptions: {
+			globals: {
+				...globals.browser,
+				...globals.es2015,
+				...globals.node,
+				...sharedGlobals,
+			},
+			parser: babelEslintParser,
+			parserOptions: { ...sharedParserOptions },
+		},
+		settings: { ...sharedSettings },
+		rules: { ...sharedRules },
+	},
+];
+
+module.exports = { ...legacyConfig, flat: flatConfig };