Migrate to ESLint flat config (#519)

.vscode/settings.json

@@ -0,0 +1,6 @@
+{
+  "eslint.experimental.useFlatConfig": true,
+  "eslint.options": {
+    "overrideConfigFile": "eslint.config.mjs"
+  }
+}

eslint.config.mjs

@@ -0,0 +1,153 @@
+// @ts-check
+
+import eslint from '@eslint/js';
+import eslintConfigPrettier from 'eslint-config-prettier';
+import eslintPluginJest from 'eslint-plugin-jest';
+import * as eslintPluginJestFormatting from 'eslint-plugin-jest-formatting';
+import eslintPluginUnicorn from 'eslint-plugin-unicorn';
+import typescriptEslint from 'typescript-eslint';
+
+export default typescriptEslint.config(
+  eslint.configs.recommended,
+  ...typescriptEslint.configs.strictTypeChecked,
+  ...typescriptEslint.configs.stylisticTypeChecked,
+  /** @type {any} */ (eslintPluginUnicorn.configs['flat/recommended']),
+  eslintConfigPrettier,
+  {
+    languageOptions: {
+      ecmaVersion: 'latest',
+      parserOptions: {
+        project: true,
+      },
+    },
+    rules: {
+      // Core ESLint rules
+      'accessor-pairs': 'error',
+      'camelcase': [
+        'error',
+        {
+          allow: ['Vue2_6', 'Vue2_7'],
+        },
+      ],
+      'consistent-return': 'error',
+      'curly': ['error', 'all'],
+      'dot-notation': 'error',
+      'eqeqeq': 'error',
+      'getter-return': 'error',
+      'grouped-accessor-pairs': ['error', 'getBeforeSet'],
+      'guard-for-in': 'error',
+      'no-array-constructor': 'error',
+      'no-bitwise': 'error',
+      'no-constant-binary-expression': 'error',
+      'no-constant-condition': 'error',
+      'no-else-return': ['error', { allowElseIf: false }],
+      'no-irregular-whitespace': 'error',
+      'no-lonely-if': 'error',
+      'no-loop-func': 'error',
+      'no-mixed-operators': 'error',
+      'no-new-object': 'error',
+      'no-prototype-builtins': 'error',
+      'no-return-assign': 'error',
+      'no-return-await': 'error',
+      'no-shadow': 'off', // replaced by @typescript-eslint/no-shadow
+      'no-template-curly-in-string': 'error',
+      'no-unsafe-optional-chaining': [
+        'error',
+        { disallowArithmeticOperators: true },
+      ],
+      'object-shorthand': ['error', 'always', { avoidQuotes: true }],
+      'prefer-arrow-callback': 'error',
+      'prefer-template': 'error',
+      'quotes': [
+        'error',
+        'single',
+        { avoidEscape: true, allowTemplateLiterals: false },
+      ],
+      'radix': 'error',
+      'spaced-comment': 'error',
+
+      // eslint-plugin-unicorn
+      'unicorn/filename-case': 'off',
+      'unicorn/no-null': 'off',
+      'unicorn/no-useless-undefined': 'off', // conflicts with consistent-return
+      'unicorn/prevent-abbreviations': [
+        'warn',
+        {
+          replacements: {
+            prop: false,
+          },
+        },
+      ],
+
+      // @typescript-eslint/eslint-plugin
+      '@typescript-eslint/consistent-type-exports': 'error',
+      '@typescript-eslint/consistent-type-imports': 'error',
+      '@typescript-eslint/explicit-module-boundary-types': 'error',
+      '@typescript-eslint/no-confusing-void-expression': [
+        'error',
+        {
+          ignoreVoidOperator: true,
+          ignoreArrowShorthand: true,
+        },
+      ],
+      '@typescript-eslint/no-explicit-any': 'off', // needed for Vue types compatibility
+      '@typescript-eslint/no-extraneous-class': 'off',
+      '@typescript-eslint/no-shadow': [
+        'warn',
+        { ignoreOnInitialization: true },
+      ],
+      '@typescript-eslint/no-unsafe-unary-minus': 'error',
+      '@typescript-eslint/no-useless-template-literals': 'error',
+      '@typescript-eslint/prefer-enum-initializers': 'error',
+      '@typescript-eslint/prefer-readonly': 'error',
+      '@typescript-eslint/promise-function-async': 'error',
+      '@typescript-eslint/sort-type-constituents': 'warn',
+      '@typescript-eslint/switch-exhaustiveness-check': 'error',
+    },
+  },
+  { ignores: ['**/*.json', 'node_modules', 'dist'] },
+  {
+    files: ['eslint.config.mjs'],
+    rules: {
+      // less strict rules for ESLint config while some ESLint plugins don't provide proper types
+      '@typescript-eslint/no-unsafe-argument': 'off',
+      '@typescript-eslint/no-unsafe-assignment': 'off',
+      '@typescript-eslint/no-unsafe-call': 'off',
+      '@typescript-eslint/no-unsafe-member-access': 'off',
+    },
+  },
+  {
+    files: ['tests/**/*.spec.ts'],
+    plugins: {
+      'jest': eslintPluginJest,
+      'jest-formatting': eslintPluginJestFormatting,
+    },
+    rules: {
+      ...eslintPluginJest.configs['flat/recommended'].rules,
+      ...eslintPluginJestFormatting.configs.recommended.overrides[0].rules,
+
+      // additional Jest rules
+      'jest/consistent-test-it': 'error',
+      'jest/no-confusing-set-timeout': 'error',
+      'jest/no-duplicate-hooks': 'error',
+      'jest/no-test-return-statement': 'error',
+      'jest/no-untyped-mock-factory': 'error',
+      'jest/prefer-called-with': 'warn',
+      'jest/prefer-comparison-matcher': 'warn',
+      'jest/prefer-equality-matcher': 'warn',
+      'jest/prefer-expect-resolves': 'warn',
+      'jest/prefer-hooks-on-top': 'warn',
+      'jest/prefer-mock-promise-shorthand': 'error',
+      'jest/prefer-spy-on': 'warn',
+      'jest/prefer-strict-equal': 'warn',
+      'jest/require-to-throw-message': 'warn',
+      'jest/require-top-level-describe': 'warn',
+
+      // less strict other rules
+      '@typescript-eslint/no-non-null-assertion': 'off',
+      '@typescript-eslint/no-unsafe-argument': 'off',
+      '@typescript-eslint/no-unsafe-assignment': 'off',
+      '@typescript-eslint/no-unsafe-call': 'off',
+    },
+  },
+);