Is the config file .eslintrc deprecated in favor of .eslintrc.* files?

Yes, but it's still supported.

What format should we use instead?

.eslintrc.json is the most similar to what you're used to, it's the same format but with an explicit JSON extension. If you'd like to avoid extra files, you can alternatively add your config to the eslintConfig field of package.json. If you need dynamic code, use .eslintrc.js or .eslintrc.cjs.

Configuration Guide

I had this problem too.

Previously, I would install ES Lint and then create the eslintrc.json config file in two steps:

npm install eslint --save-dev npm init @eslint/config

But that now caused the error you got.

If I ran only one command: npm init @eslint/config@latest it walked me through the set-up steps and created the eslint.config.js file I needed.

First thing to note is that the eslint.config.js file should be at the root level of your project; you can target specific files and folders within the project structure from this one central configuration if need be i.e. in a monorepo.

The "extends" property does not exist under the new ESLint flat configuration schema, as per the 'Configuration Objects' specified in their documentation. You can directly import a configuration and declare it within the new eslint.config.js, however if that configuration extends other configurations this won't work - I can't see your '@ui' configuration definition so I have assumed in my code block that it does not extend configurations within itself (if it does, see blog link below on using @eslint/eslintrc 'compat' to bring in such configurations).

The "parserOptions" is also not a top-level property under the new schema and falls inside the 'languageOptions' property.

    // eslint.config.js at the project root
    import ParserTypescriptEslint from '@typescript-eslint/parser'
    import PluginImport from 'eslint-plugin-import';
    import PluginJest from 'eslint-plugin-jest';
    import UIConfig from 'ui-config-path';
    import globals from 'globals';
    
    export default [
      UIConfig,
      {
        files: [ "// Specify the files you wish to target with this configuration" ],
        languageOptions: {
          ecmaVersion: 2021,
          sourceType: 'module',
          globals: {
            ...globals.browser,
            ...globals.es2021,
          },
          parser: ParserTypescriptEslint,
          parserOptions: {
            project: ["./tsconfig.json"],
            tsconfigRootDir: __dirname,
          },
        },
        plugins: {
          import: PluginImport,
          jest: PluginJest
        },
        rules: {
          "jest/no-deprecated-functions": "off"
        },
        settings: {
          "import/resolver": {
            ...PluginImport.configs.typescript.settings['import/resolver'],
            typescript: {
              project: ["tsconfig.json"],
            },
          },
        },
        
      }
    ]

If you have multiple sub-configurations that you wish to use for different folders/systems - React, Express, Jest, etc - then you can write those configurations following the new schema and spread them into the main eslint.config.js file. This is a snippet of my main eslint.config.js that uses several different sub-configurations.

    import { EslintConfig, ConfigPrettier } from '@packages/eslint-config';
    import { EslintConfigReact } from '@packages/eslint-config-react';
    import { EslintConfigReactTest } from '@packages/eslint-config-react-test';
    import { EslintConfigExpress } from '@packages/eslint-config-express';
    
    export default [
      ConfigPrettier,
      {
        ignores: ['**/node_modules', '**/dist', '**/build', '**/__snapshots__', '**/mocks', '**/coverage'],
      },
      {
        // Client - React
        files: [
          'apps/*/frontend/**/*.ts',
          'apps/*/frontend/**/*.tsx',
          'apps/*/frontend/**/*.jsx',
          'apps/*/frontend/**/*.js',
        ],
        languageOptions: { ...EslintConfig.languageOptions, ...EslintConfigReact.languageOptions },
        plugins: { ...EslintConfig.plugins, ...EslintConfigReact.plugins },
        rules: { ...EslintConfig.rules, ...EslintConfigReact.rules },
        settings: { ...EslintConfig.settings, ...EslintConfigReact.settings },
      }
    ];

The following blog post was useful to me in setting up the new ESLint flat configuration in a monorepo structure: migration-eslint-to-flat-config