[eslint] Config (unnamed): Unexpected key "0" found.

[daniel-stoian-lgp]

Bug report

Hello,
After migrating to eslint 9 with flat config, the Eslint webpack plugin throws the following error:
[eslint] Config (unnamed): Unexpected key "0" found.

Actual Behavior

[eslint] Config (unnamed): Unexpected key "0" found.

Expected Behavior

No error should be shown

How Do We Reproduce?

Former configuration for eslintrc

		new ESLintPlugin({
			// Plugin options
			extensions: ['js', 'mjs', 'jsx', 'ts', 'tsx'],
			formatter: require.resolve('react-dev-utils/eslintFormatter'),
			eslintPath: require.resolve('eslint'),
			// ESLint class options
			resolvePluginsRelativeTo: __dirname,
			// @remove-on-eject-begin
			baseConfig: {
				extends: [require.resolve('eslint-config-enact/index.js')],
				rules: {
					...(!hasJsxRuntime && {
						'react/jsx-uses-react': 'warn',
						'react/react-in-jsx-scope': 'warn'
					})
				}
			},
			useEslintrc: false,
			// @remove-on-eject-end
			cache: true
		})

Current config for flat config:

		const eslintConfig = require('eslint-config-enact/index.js');

		new ESLintPlugin({
			// Plugin options
			configType: 'flat',
			extensions: ['js', 'mjs', 'jsx', 'ts', 'tsx'],
			formatter: require.resolve('react-dev-utils/eslintFormatter'),
			eslintPath: require.resolve('eslint'),
			// @remove-on-eject-begin
			baseConfig: [{
				...eslintConfig,
				rules: {
					...(!hasJsxRuntime && {
						'react/jsx-uses-react': 'warn',
						'react/react-in-jsx-scope': 'warn'
					})
				}
			}],
			overrideConfigFile: true,
			// @remove-on-eject-end
			cache: true
		})

The index.js looks like this:

	const eslint = require('@eslint/js');

	module.exports = [
		eslint.configs.recommended,
		{
			ignores: [
				'**/node_modules/',
				'**/node_modules/*',
				'**/build/',
				'**/build/*'
			]
		}
	];

I checked the API reference and the configuration should work https://eslint.org/docs/latest/integrate/nodejs-api#-new-eslintoptions . The configuration is pretty basic, it only reuses eslint recommended rules and ignores certain paths.

Eslint command using the above configuration works well. No errors.
When I try to build an app with webpack, I get the error mentioned in the description.

Please paste the results of npx webpack-cli info here, and mention other relevant information

[evenstensberg]

Please format your code better. Right now it's very unreadable.

[daniel-stoian-lgp]

Please format your code better. Right now it's very unreadable.

done, sorry for that

[alexander-akait]

Do you use flat config?

[daniel-stoian-lgp]

Do you use flat config?

Yes. Eslint 9.19. Flat config is enabled by default
Eslint command using the above configuration works well. No errors.
When I try to build an app with webpack, I get the error mentioned in the description.

[alexander-akait]

Please set https://github.com/webpack-contrib/eslint-webpack-plugin?tab=readme-ov-file#configtype to flat, we are preparing the next major release with flat by default - #272

[daniel-stoian-lgp]

Please set https://github.com/webpack-contrib/eslint-webpack-plugin?tab=readme-ov-file#configtype to flat, we are preparing the next major release with flat by default - #272

I already did that

[alexander-akait]

@daniel-stoian-lgp Can you create reproducible test repo?

[daniel-stoian-lgp]

I have to close the issue, as it does not seem to be caused by eslint-webpack-plugin.
In Eslint 9, The configData object which is used in options.baseConfig does not seem to allow extensions from other configurations anymore https://eslint.org/docs/latest/use/configure/ I came to this conclusion as nothing is mentioned about extending configurations.
In Eslint 8, it specifically mentioned the Extentending configuration files part: https://eslint.org/docs/v8.x/use/configure/
I managed to solve my issue by specifiying a new config file in options.overrideConfigFile