ESLint 9.0: Mandatory parserOptions.project for TypeScript rules

The requirement for parserOptions.project is not imposed by ESLint 9.0 itself, but by @typescript-eslint when type-aware rules are enabled. These rules (like @typescript-eslint/no-unused-vars with type checking) require a TypeScript program to function, which cannot be initialized without a valid tsconfig.json.

In mixed-language monorepos, the error occurs because a global configuration attempts to apply type-aware rules to JavaScript files that lack a TypeScript configuration. To resolve this, you must decouple your general linting from type-aware analysis.

Recommended Strategy: Flat Config Segmentation

Using ESLint's flat config (eslint.config.js), you should define multiple configuration objects. This allows you to apply type-aware rules only to files that have a project structure supporting them.

• Define a Base Configuration: Create an object for all files (JS and TS) that includes standard rules which do not require type information.
• Define a Type-Aware Configuration: Create a separate object that targets only **/*.ts and **/*.tsx files. This is where you define parserOptions.project.
• Use project: true: Instead of hardcoding paths, setting project: true allows the parser to automatically find the nearest tsconfig.json for each file, which significantly reduces maintenance.

Example Configuration

import js from "@eslint/js";
import tsPlugin from "@typescript-eslint/eslint-plugin";

export default [\n  // 1. Global settings for all files (No type-aware)
  {
    files: ["**/*.js", "**/*.jsx"],\n    extends: [js.configs.recommended],
    rules: {
      "no-console": "warn"
    }
  },
n
  // 2. Type-aware settings for TypeScript only
  {
    files: ["**/*.ts", "**/*.tsx"],
    plugins: {
      "@typescript-eslint": tsPlugin
    },
    parser: "@typescript-eslint/parser",
    parserOptions: {
      project: true, // Auto-discovers tsconfig.json
    },
    rules: {
      ...tsPlugin.configs.recommendedTypeChecked.rules,
      "@typescript-eslint/no-unused-vars": "error"
    }
  }
];

Handling Packages without tsconfig.json

If a package contains TypeScript files but lacks a tsconfig.json, you have two viable paths:

1. Create a Minimal Config: Add a tsconfig.json to the directory with just {"compilerOptions": {"include": ["*"]}}. This satisfies the parser without requiring complex build logic.
2. Exclude from Type-Aware Rules: If you do not need type-aware linting for those files, ensure they are not caught by the glob in the configuration object that defines parserOptions.project.

Assumption: You are using the ESLint 9+ flat config format and @typescript-eslint version 7 or 8.