Prerequisites

• I'm using the latest version
• I've read the relevant documentation
• I've searched for existing issues
• I've checked the list of known issues
• I've read the issue reproduction guide

Reproduction url

https://codesandbox.io/p/devbox/yjgd5f

Reproduction access

• I've made sure the reproduction is publicly accessible

Description of the issue

With a recent end-of-life for ESLint v8, more people are migrating to FlatConfig. New eslint.config.js file is pretty standard, so most imports can be tracked without any special tricks. However, if this file refers to a config that is still relying on an old config system, Knip fails to detect that a project dependency is used.

Here is an example for eslint-config-next:

.eslintrc.json before conversion

{
  "extends": "next/core-web-vitals"
}

Knip is smart enough to know that next/core-web-vitals comes from eslint-config-next, so does not mark this dependency as unused.

eslint.config.js after conversion

npx @eslint/migrate-config .eslintrc.json

↓

import path from "node:path";
import { fileURLToPath } from "node:url";
import js from "@eslint/js";
import { FlatCompat } from "@eslint/eslintrc";

const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);
const compat = new FlatCompat({
  baseDirectory: __dirname,
  recommendedConfig: js.configs.recommended,
  allConfig: js.configs.all,
});

export default [...compat.extends("next/core-web-vitals")];

Knip does not know about compat.extends("next/core-web-vitals"), so marks eslint-config-next as unused in package.json.

Note that in the real world scenario the call may also look like this:

...flatCompat.extends('next/core-web-vitals', 'next/typescript')

In the long term most popular community plugins and configs will be converted to FlatConfig so will be imported natively using ESM. But it would be great if Knip could handle compat.extends in the meantime to help folks with migration.