Flat eslint.config.js (package.type="commonjs")

eslint.config.js

const {
  DEFAULT_IGNORES,
  setEslintLanguageOptionsBrowser,
  setEslintPluginJest,
  setEslintPluginJestDom,
  setEslintPluginPrettier,
  setEslintPluginTypescriptEslint,
  setEslintPluginUnicorn,
} = require('./eslint.config.utils.js');

const allowList = ['obj', 'tmp', 'args', 'props', 'OTimerGetTimesArgs'];

module.exports = [
  { ignores: DEFAULT_IGNORES },
  setEslintLanguageOptionsBrowser(),
  setEslintPluginUnicorn({ allowList }),
  setEslintPluginJest(),
  setEslintPluginJestDom(),
  setEslintPluginPrettier(),
  ...setEslintPluginTypescriptEslint(),
  
  // NOTE: Allow main js-files to be as common-js (not module-js)
  {
    files: ['*.js'],
    rules: {
      'no-undef': 'off',
      '@typescript-eslint/no-require-imports': 'off',
    },
  },
];

eslint.config.utils.js

const pluginJs = require('@eslint/js');
const globals = require('globals');
const tseslint = require('typescript-eslint');
const eslintConfigPrettier = require('eslint-config-prettier');
const eslintPluginPrettier = require('eslint-plugin-prettier/recommended');
const eslintPluginJest = require('eslint-plugin-jest');
const eslintPluginJestDom = require('eslint-plugin-jest-dom');

//

const DEFAULT_IGNORES = ['coverage/*', 'dist/*', 'tmp.js', '**/*.test.js', '**/*.cjs'];

//

function setEslintLanguageOptionsBrowser() {
  return { languageOptions: { globals: globals.browser } };
}

//

function setUnicornAllowList(list = []) {
  return {
    allowList: list.reduce((list, key) => {
      list[key] = true;
      return list;
    }, {}),
  };
}

function setEslintPluginUnicorn({ rules, allowList } = {}) {
  return {
    ...eslintPluginUnicorn.configs['flat/recommended'],
    rules: {
      ...eslintPluginUnicorn.configs['flat/recommended'].rules,
      'unicorn/switch-case-braces': ['error', 'avoid'],
      'unicorn/prefer-string-replace-all': 'off',
      'unicorn/no-nested-ternary': 'off',
      'unicorn/no-array-reduce': 'off',
      'unicorn/prefer-at': 'off',
      'unicorn/no-null': 'off',
      'unicorn/filename-case': ['error', { cases: { kebabCase: true, pascalCase: true } }],
      'unicorn/prevent-abbreviations': ['error', setUnicornAllowList(allowList)],
      ...rules,
    },
  };
}

//

function setEslintPluginJest({ rules } = {}) {
  return {
    ...eslintPluginJest.configs['flat/recommended'],
    rules: {
      ...eslintPluginJest.configs['flat/recommended'].rules,
      ...rules,
    },
  };
}

function setEslintPluginJestDom({ rules } = {}) {
  return {
    ...eslintPluginJestDom.configs['flat/recommended'],
    rules: {
      ...eslintPluginJestDom.configs['flat/recommended'].rules,
      ...rules,
    },
  };
}

//

function setEslintPluginPrettier({ rules } = {}) {
  return {
    ...eslintPluginPrettier,
    rules: {
      ...eslintConfigPrettier.rules,
      ...rules,
    },
  };
}

//

function setEslintPluginTypescriptEslint({ rules } = {}) {
  return [
    { ...pluginJs.configs.recommended },
    ...tseslint.configs.recommended,

    {
      rules: {
        'max-params': ['error', 4],
        'no-unused-vars': 'off',
        '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_', varsIgnorePattern: '^_' }],
        '@typescript-eslint/no-var-requires': 'off',
        ...rules,
      },
    },
  ];
}

//

module.exports = {
  DEFAULT_IGNORES,
  setEslintLanguageOptionsBrowser,
  setEslintPluginUnicorn,
  setEslintPluginJest,
  setEslintPluginJestDom,
  setEslintPluginPrettier,
  setEslintPluginTypescripEslint,
};