/shared-tooling-eslint-prettier

name: shared-tooling-eslint-prettier
description: ESLint 9/10 flat config with defineConfig(), Prettier v3.0+ shared config, eslint-config-prettier integration, typescript-eslint v8+ projectService

ESLint & Prettier

> **Quick Guide:** ESLint 9+ flat config with `defineConfig()` and `globalIgnores()`. typescript-eslint v8+ with `projectService: true`. Prettier shared config with consistent formatting. eslint-config-prettier to disable conflicting rules. eslint-plugin-only-warn for better DX. > > **WARNING**: ESLint 10 was released February 2026 and completely removes .eslintrc support. Migrate to flat config now.

---

<critical_requirements>

CRITICAL: Before Using This Skill

> **All code must follow project conventions in CLAUDE.md** (kebab-case, named exports, import ordering, `import type`, named constants)

**(You MUST use ESLint 9+ flat config with `defineConfig()` from `eslint/config` - NOT legacy .eslintrc)**

**(You MUST use `globalIgnores()` for explicit global ignore patterns - NOT bare `ignores` property)**

**(You MUST use typescript-eslint v8+ with `projectService: true` for typed linting)**

**(You MUST include eslint-plugin-only-warn to convert errors to warnings for better DX)**

**(You MUST use eslint-config-prettier to disable formatting rules that conflict with Prettier)**

</critical_requirements>

---

**Auto-detection:** ESLint 9 flat config, defineConfig, globalIgnores, eslint.config.ts, Prettier config, prettier.config.mjs, eslint-config-prettier, eslint-plugin-only-warn, typescript-eslint projectService, .eslintrc migration

**When to use:**

• Setting up ESLint 9/10 flat config (standalone or shared)
• Configuring Prettier with shared config
• Migrating from legacy .eslintrc to flat config
• Integrating ESLint and Prettier (eslint-config-prettier)
• Configuring typescript-eslint v8+ with projectService
• Setting up custom ESLint rules (named exports, import restrictions, type imports)

**When NOT to use:**

• Runtime code (this is build-time tooling only)
• CI/CD pipeline configuration
• Git hooks or lint-staged setup
• TypeScript compiler configuration (tsconfig)
• Bundler configuration

**Detailed Resources:**

• [examples/core.md](examples/core.md) - Essential flat config + Prettier setup
• [examples/eslint.md](examples/eslint.md) - Advanced ESLint patterns (shared configs, custom rules, ESLint 10 migration)
• [examples/prettier.md](examples/prettier.md) - Advanced Prettier patterns (TS config, experimental options, ignore files)
• [reference.md](reference.md) - Version reference and official documentation links

---

<philosophy>

Philosophy

Linting and formatting should be **fast, consistent, and non-blocking**. Developers should not fight with tools - tools should help catch issues early while staying out of the way during development.

**Core principles:**

1. **Warnings, not errors** - Use only-warn to convert lint errors to warnings so developers can iterate without being blocked 2. **Single source of truth** - Shared configs prevent drift between packages and team members 3. **Automate formatting** - Prettier handles all formatting decisions; ESLint handles code quality only 4. **No conflicts** - eslint-config-prettier ensures ESLint and Prettier never disagree

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: ESLint 9+ Flat Config with defineConfig()

ESLint 9+ uses flat config. The `defineConfig()` helper from `eslint/config` provides type safety and automatic array flattening. `globalIgnores()` explicitly marks global ignore patterns.

// eslint.config.ts — the essential flat config
import js from "@eslint/js";
import { defineConfig, globalIgnores } from "eslint/config";
import tseslint from "typescript-eslint";
import eslintConfigPrettier from "eslint-config-prettier";
import * as onlyWarnPlugin from "eslint-plugin-only-warn";

export default defineConfig(
  globalIgnores(["dist/**", "generated/**", "node_modules/**"]),
  js.configs.recommended,
  eslintConfigPrettier,
  tseslint.configs.recommended,
  { plugins: { "only-warn": onlyWarnPlugin } }, // must be last
);

**Key points:** `defineConfig()` auto-flattens arrays (no spread operators needed), `globalIgnores()` prevents ambiguous behavior of bare `ignores`, only-warn converts all preceding errors to warnings, eslint-config-prettier disables formatting rules that conflict with Prettier.

See [examples/core.md](examples/core.md) for the complete standalone and shared config patterns.

---

Pattern 2: typescript-eslint v8+ with projectService

The `projectService` feature (stable since v8) auto-discovers the nearest tsconfig.json for each file, replacing the fragile manual `project` path.

parserOptions: {
  projectService: true,
  allowDefaultProject: ["*.config.ts", "*.config.mjs"],
},

**Key points:** Eliminates need for tsconfig.eslint.json files, faster than manual project configuration, `allowDefaultProject` handles config files not in tsconfig.

See [examples/core.md](examples/core.md) for full typescript-eslint configuration.

---

Pattern 3: Shared Config Pattern (Monorepos/Teams)

For teams or monorepos, extract linting config into a shared package to prevent drift.

// packages/eslint-config/base.ts — shared config
export const baseConfig = defineConfig(
  globalIgnores(["dist/**", "generated/**"]),
  js.configs.recommended,
  eslintConfigPrettier,
  tseslint.configs.recommended,
  { plugins: { "only-warn": onlyWarnPlugin } },
);

// apps/my-app/eslint.config.ts — consuming shared config
export default defineConfig(baseConfig, customRules, {
  rules: { "no-console": "warn" },
});

**Key points:** `defineConfig()` auto-flattens so no spread operators needed, single source of truth prevents config drift, TypeScript config file for type checking.

See [examples/eslint.md](examples/eslint.md) for shared config and custom rules patterns.

---

Pattern 4: Prettier Shared Config

Prettier co