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.

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<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>

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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 - Essential flat config + Prettier setup
• examples/eslint.md - Advanced ESLint patterns (shared configs, custom rules, ESLint 10 migration)
• examples/prettier.md - Advanced Prettier patterns (TS config, experimental options, ignore files)
• 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 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 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 for shared config and custom rules patterns.

---

Pattern 4: Prettier Shared Config

Prettier config should be consistent across all packages. Use a shared config for teams.

// prettier.config.mjs
const config = {
  printWidth: 100,
  semi: true,
  singleQuote: false,
  bracketSpacing: true,
  arrowParens: "always",
  endOfLine: "lf",
  bracketSameLine: false,
  // trailingComma: "all" is the default in Prettier 3.0+
};
export default config;

Key points: trailingComma: "all" is the default in v3.0+ (don't set it explicitly), bracketSameLine replaces deprecated jsxBracketSameLine, explicit endOfLine: "lf" prevents cross-platform issues.

See examples/prettier.md for TypeScript config files, experimental options, and ignore patterns.

---

Pattern 5: Custom ESLint Rules

Common custom rules for enforcing project conventions — named exports, consistent type imports, unused variable detection, and import boundary enforcement:

export const customRules = {
  rules: {
    "import/no-default-export": "warn",
    "@typescript-eslint/consistent-type-imports": [
      "warn",
      { prefer: "type-imports" },
    ],
    "@typescript-eslint/no-unused-vars": [
      "warn",
      { argsIgnorePattern: "^_", varsIgnorePattern: "^_" },
    ],
  },
};

See examples/eslint.md for the full custom rules pattern including import boundary restrictions.

---

Pattern 6: Using extends Property (ESLint 9.15+)

The extends property in flat config objects simplifies plugin composition — standardizes config merging regardless of plugin format (object, array, or string):

export default defineConfig({
  files: ["**/*.ts", "**/*.tsx"],
  extends: [
    "eslint/recommended",
    tseslint.configs.recommended,
    // Add your framework plugin's flat config here
  ],
  rules: { "no-console": "warn" },
});

See examples/core.md for the full extends pattern.

</patterns>

---

<decision_framework>

Decision Framework

ESLint + Prettier vs Alternatives

Need linting and formatting?
├─ Speed is critical bottleneck (1000+ files)?
│   └─ YES → Consider a Rust-based unified linter/formatter (not this skill's scope)
└─ Need mature plugin ecosystem?
    └─ YES → ESLint 9/10 + Prettier ✓

ESLint + Prettier strengths: Mature ecosystem, extensive plugin support, framework-specific plugins

ESLint 9 vs ESLint 10

Which ESLint version?
├─ New project?
│   └─ ESLint 10 (latest, cleanest API)
├─ Existing project with flat config?
│   └─ ESLint 10 (straightforward upgrade)
├─ Existing project with .eslintrc?
│   ├─ Can invest time to migrate?
│   │   └─ YES → Migrate to flat config, then ESLint 10
│   └─ NO → ESLint 9.x (still supported, but plan migration)
└─ Node.js < 20.19.0?
    └─ ESLint 9.x (ESLint 10 requires 20.19.0+)

Shared Config vs Local Config

Setting up linting/formatting?
├─ Monorepo with multiple packages?
│   └─ YES → Shared config ✓
├─ Team project (2+ developers)?
│   └─ YES → Shared config (consistency matters)
└─ Single package / solo project?
    └─ YES → Local config is fine

Prettier Config File Format

What Prettier config format to use?
├─ Need type checking in config?
│   ├─ Node.js 22.6.0+? → prettier.config.ts
│   └─ NO → Use .mjs with JSDoc types
├─ ESM project? → prettier.config.mjs
└─ CommonJS project? → prettier.config.cjs

</decision_framework>

---

<red_flags>

RED FLAGS

High Priority Issues:

• ❌ Using legacy .eslintrc format instead of ESLint 9+ flat config (BROKEN in ESLint 10)
• ❌ Using bare ignores property instead of globalIgnores() helper (ambiguous behavior — acts as global ignores when alone, but as local excludes when paired with other properties)
• ❌ Missing eslint-plugin-only-warn (errors block developers during development)
• ❌ Missing eslint-config-prettier (ESLint and Prettier rules conflict, creating endless fix cycles)

Medium Priority Issues:

• ⚠️ Using manual project option instead of projectService: true in typescript-eslint (fragile in monorepos, slower)
• ⚠️ Using deprecated jsxBracketSameLine option in Prettier (renamed to bracketSameLine in Prettier 2.4)
• ⚠️ Explicitly setting trailingComma: "all" in Prettier 3.0+ (it is already the default)
• ⚠️ Using tseslint.config() wrapper (planned for deprecation in favor of ESLint's native defineConfig())
• ⚠️ Hardcoded config values in each package instead of shared config

Gotchas & Edge Cases:

• only-warn plugin must be loaded AFTER other plugins to convert their errors to warnings
• defineConfig() auto-flattens arrays — never use spread operators with it
• projectService requires typescript-eslint v8+ (was EXPERIMENTAL_useProjectService in v6-v7)
• ESLint 10 requires Node.js ^20.19.0 || ^22.13.0 || >=24 (v21.x and v23.x explicitly unsupported)
• ESLint 10 config lookup starts from linted file directory (not cwd) — enables monorepo multi-config
• Prettier TypeScript config files (.prettierrc.ts) require Node.js 22.6.0+; before Node v24.3.0 run with --experimental-strip-types
• Prettier 3.0+ APIs are async — plugins using sync APIs need migration (use @prettier/sync for sync wrappers)
• ESLint 9.15+: eslint.config.ts TypeScript config files and extends property supported natively
• ESLint 9.34+: Multithreaded linting available via --concurrency flag (30-300% performance boost on large projects)
• ESLint 10: /* eslint-env */ comments now trigger errors; deprecated Linter methods removed

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(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)

Failure to follow these rules will cause inconsistent tooling, conflicting formatting rules, and blocked developers.

WARNING: ESLint 10 (February 2026) completely removes .eslintrc support. Plan migration now.

</critical_reminders>