ESLint v9 Next.js

eslint.config.js

import path from 'path';
import { fileURLToPath } from 'url';

import comments from '@eslint-community/eslint-plugin-eslint-comments/configs';
import { fixupConfigRules } from '@eslint/compat';
import { FlatCompat } from '@eslint/eslintrc';
import js from '@eslint/js';
import eslintConfigPrettier from 'eslint-config-prettier';
import jsdoc from 'eslint-plugin-jsdoc';
import * as regexpPlugin from 'eslint-plugin-regexp';
import pluginSecurity from 'eslint-plugin-security';
import tseslint from 'typescript-eslint';

const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);

const compat = new FlatCompat({
  baseDirectory: __dirname,
  resolvePluginsRelativeTo: __dirname,
});

export default tseslint.config(
  {
    ignores: ['.next'],
  },

  js.configs.recommended,
  ...tseslint.configs.strictTypeChecked,
  ...tseslint.configs.stylisticTypeChecked,
  ...fixupConfigRules(compat.extends('plugin:@next/next/recommended')),
  ...fixupConfigRules(compat.extends('plugin:react/recommended')),
  ...fixupConfigRules(compat.extends('plugin:react-hooks/recommended')),
  ...fixupConfigRules(compat.extends('plugin:tailwindcss/recommended')),
  ...fixupConfigRules(compat.extends('plugin:jsx-a11y/strict')),
  comments.recommended,
  regexpPlugin.configs['flat/recommended'],
  jsdoc.configs['flat/recommended-typescript-error'],
  pluginSecurity.configs.recommended,
  eslintConfigPrettier,

  {
    linterOptions: {
      reportUnusedDisableDirectives: true,
    },
    languageOptions: {
      parserOptions: {
        project: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
    settings: {
      react: {
        version: 'detect',
      },
    },
    rules: {
      '@typescript-eslint/no-unused-vars': [
        'error',
        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
      ],

      '@typescript-eslint/consistent-type-imports': [
        'warn',
        { prefer: 'type-imports', fixStyle: 'separate-type-imports' },
      ],

      '@typescript-eslint/no-misused-promises': [
        'error',
        { checksVoidReturn: { attributes: false } },
      ],

      '@typescript-eslint/no-unnecessary-condition': [
        'error',
        {
          allowConstantLoopConditions: true,
        },
      ],

      'react/react-in-jsx-scope': 'off',

      // prettier figures out the lines between tags
      'jsdoc/tag-lines': 'off',

      // we use some type tags in js files
      'jsdoc/check-tag-names': ['error', { typed: false }],
    },
  },
);

eslint.d.ts

/**
 * Since ESLint and many plugins are written in JavaScript, we need to provide
 * or change some types to make them work with TypeScript.
 */

// From: https://github.com/t3-oss/create-t3-turbo/blob/main/tooling/eslint/types.d.ts
declare module '@eslint/js' {
  import type { Linter } from 'eslint';

  export const configs: {
    readonly recommended: { readonly rules: Readonly<Linter.RulesRecord> };
    readonly all: { readonly rules: Readonly<Linter.RulesRecord> };
  };
}

declare module '@eslint-community/eslint-plugin-eslint-comments/configs' {
  import type { Linter } from 'eslint';

  export const recommended: {
    rules: Linter.RulesRecord;
  };
}

declare module '@eslint/eslintrc' {
  import type { Linter } from 'eslint';

  export class FlatCompat {
    constructor({
      baseDirectory,
      resolvePluginsRelativeTo,
    }: {
      baseDirectory: string;
      resolvePluginsRelativeTo: string;
    });

    extends(extendsValue: string): Linter.FlatConfig & {
      [Symbol.iterator]: () => IterableIterator<Linter.FlatConfig>;
    };
  }
}

declare module '@eslint/compat' {
  import type { Linter } from 'eslint';
  import type { ConfigWithExtends } from 'typescript-eslint';

  export const fixupConfigRules: (
    config: string | Linter.FlatConfig,
  ) => ConfigWithExtends[];
}

declare module 'eslint-plugin-regexp' {
  import type { Linter } from 'eslint';
  import type { ConfigWithExtends } from 'typescript-eslint';

  export const configs: {
    'flat/recommended': {
      [Symbol.iterator]: () => IterableIterator<ConfigWithExtends>;
      rules: Linter.RulesRecord;
    };
    'flat/all': {
      [Symbol.iterator]: () => IterableIterator<ConfigWithExtends>;
      rules: Linter.RulesRecord;
    };
  };
}

declare module 'eslint-plugin-security' {
  import type { Linter } from 'eslint';
  import type { ConfigWithExtends } from 'typescript-eslint';

  export const configs: {
    recommended: {
      [Symbol.iterator]: () => IterableIterator<ConfigWithExtends>;
      rules: Linter.RulesRecord;
    };
  };
}