ESLint 9 error 'ESLint couldn't find an eslint.config.(js|mjs|cjs) file' after upgrading from ESLint 8. How do I migrate from .eslintrc?

Cause: Starting with ESLint v9.0.0, the "flat config" format is the default and the old .eslintrc.* files (and .eslintignore) are no longer read by default. ESLint now searches for eslint.config.js (also .mjs and .cjs, and TypeScript variants with extra setup), starting from the directory of the file being linted and moving up through parent folders. If none exists, you get this error. Your old config is still sitting there, simply ignored. Fix (best first): 1. Run the official migrator on your existing config: npx @eslint/migrate-config .eslintrc.json. It writes a starting eslint.config.mjs. Review the output; it handles most cases but not all, and it works poorly with .eslintrc.js files that contain logic. 2. Place the new file in the project root (or the package root in a monorepo) so it is found from the files you lint. 3. Convert the pieces by hand where needed: env becomes languageOptions.globals using the globals package; parserOptions become languageOptions; ignorePatterns and .eslintignore become a config object holding only an ignores array (patterns are relative to the config file; use **/ to match in any folder); dotfiles are no longer ignored by default. 4. For plugins or shared configs that still ship in the old format, install @eslint/eslintrc and wrap them with FlatCompat (compat.extends(...)) until they publish flat versions. 5. Remove leftover /* eslint-env */ comments, which are no longer recognized. 6. On ESLint 8 you can opt in early by creating eslint.config.js or setting ESLINT_USE_FLAT_CONFIG=true. Verify: npx eslint . runs without the missing-config error and reports the same rules as before; check that ignored folders such as dist are still skipped. Sources: https://eslint.org/docs/latest/use/configure/migration-guide https://eslint.org/docs/latest/use/configure/configuration-files