Eslint 9

ESLint 9 Flat Config Patterns

Overview

ESLint 9 uses flat config (eslint.config.js) as the default. The legacy .eslintrc.* format is deprecated.

Key differences from .eslintrc:

• Single config file, explicit imports — no magic extends strings.
• Config is an exported array of config objects, applied in order.
• Plugins are imported directly, not referenced by string name.
• Glob patterns in files replace the old overrides system.

Basic Setup

JavaScript

// eslint.config.js
import js from "@eslint/js";

export default [
    js.configs.recommended,
    {
        rules: {
            "no-unused-vars": "warn",
            "no-console": "warn",
        },
    },
];

TypeScript

npm install -D eslint @eslint/js typescript-eslint

// eslint.config.ts
import js from "@eslint/js";
import tseslint from "typescript-eslint";

export default tseslint.config(js.configs.recommended, tseslint.configs.recommended, {
    rules: {
        "@typescript-eslint/no-unused-vars": ["warn", { argsIgnorePattern: "^_", varsIgnorePattern: "^_" }],
    },
});

typescript-eslint exports a config() helper that provides type safety and handles array flattening.

TypeScript Config Files

ESLint 9 supports eslint.config.ts natively — no extra packages needed. Use it for full type checking of your config:

import type { Linter } from "eslint";

defineConfig

ESLint provides defineConfig for type-safe configs without typescript-eslint:

import { defineConfig } from "eslint/config";
import js from "@eslint/js";

export default defineConfig([
    js.configs.recommended,
    {
        files: ["**/*.js"],
        rules: {
            "no-console": "warn",
        },
    },
]);

defineConfig auto-flattens nested arrays and provides IntelliSense for rule names.

Adding Plugins

Plugins are imported and assigned to a namespace:

import reactPlugin from "eslint-plugin-react";
import reactHooksPlugin from "eslint-plugin-react-hooks";

export default tseslint.config(js.configs.recommended, tseslint.configs.recommended, {
    files: ["**/*.{tsx,jsx}"],
    plugins: {
        react: reactPlugin,
        "react-hooks": reactHooksPlugin,
    },
    rules: {
        "react/jsx-no-target-blank": "error",
        "react-hooks/rules-of-hooks": "error",
        "react-hooks/exhaustive-deps": "warn",
    },
    settings: {
        react: { version: "detect" },
    },
});

Many plugins now ship flat config presets. Check for plugin.configs["flat/recommended"] or similar exports.

Global Ignores

Use a config object with only ignores (no files) for global ignore patterns:

export default tseslint.config(
    { ignores: ["dist/", "node_modules/", "*.config.*", ".next/"] },
    js.configs.recommended,
    tseslint.configs.recommended,
);

Global ignores must be a standalone object — not combined with rules or files.

File-Specific Overrides

Use files globs to scope rules:

export default tseslint.config(
    js.configs.recommended,
    tseslint.configs.recommended,

    // Stricter rules for source code
    {
        files: ["src/**/*.{ts,tsx}"],
        rules: {
            "@typescript-eslint/explicit-function-return-type": "warn",
            "@typescript-eslint/no-explicit-any": "error",
        },
    },

    // Relaxed rules for tests
    {
        files: ["**/*.test.{ts,tsx}", "**/*.spec.{ts,tsx}"],
        rules: {
            "@typescript-eslint/no-explicit-any": "off",
            "@typescript-eslint/no-non-null-assertion": "off",
        },
    },

    // Config files (CommonJS)
    {
        files: ["*.config.{js,cjs,mjs}"],
        languageOptions: {
            sourceType: "commonjs",
        },
    },
);

Prettier Integration

Use eslint-config-prettier to disable formatting rules that conflict with Prettier:

import prettierConfig from "eslint-config-prettier";

export default tseslint.config(
    js.configs.recommended,
    tseslint.configs.recommended,
    prettierConfig, // must be last to override formatting rules
);

Don't use eslint-plugin-prettier — let Prettier handle formatting separately.

Common React + TypeScript Config

A complete config for a React + TypeScript project:

// eslint.config.ts
import js from "@eslint/js";
import tseslint from "typescript-eslint";
import reactPlugin from "eslint-plugin-react";
import reactHooksPlugin from "eslint-plugin-react-hooks";
import prettierConfig from "eslint-config-prettier";

export default tseslint.config(
    { ignores: ["dist/", "node_modules/", ".next/"] },

    js.configs.recommended,
    tseslint.configs.recommended,

    {
        files: ["**/*.{tsx,jsx}"],
        plugins: {
            react: reactPlugin,
            "react-hooks": reactHooksPlugin,
        },
        rules: {
            "react-hooks/rules-of-hooks": "error",
            "react-hooks/exhaustive-deps": "warn",
        },
        settings: {
            react: { version: "detect" },
        },
    },

    {
        rules: {
            "@typescript-eslint/no-unused-vars": ["warn", { argsIgnorePattern: "^_", varsIgnorePattern: "^_" }],
            "@typescript-eslint/consistent-type-imports": ["warn", { prefer: "type-imports" }],
        },
    },

    prettierConfig,
);

Migration from .eslintrc

1. Run the migration tool: npx @eslint/migrate-config .eslintrc.json
2. Review the generated eslint.config.mjs — the tool handles most conversions.
3. Replace string-based plugin references with direct imports.
4. Replace extends strings with imported config arrays.
5. Convert overrides to separate config objects with files globs.
6. Delete .eslintrc.* and .eslintignore files.

Debugging

Inspect the resolved config for any file:

npx eslint --inspect-config

This launches an interactive viewer showing which rules apply to each file and where they come from.