---
name: google-ts-styleguide
description: "Google TypeScript Style Guide reference for writing, reviewing, and refactoring TypeScript code. Use this skill whenever you are writing TypeScript code, reviewing TypeScript code, refactoring TypeScript, configuring ESLint or linter rules for TypeScript, or when the user asks about TypeScript coding conventions, naming conventions, type system best practices, or import/export patterns. Also trigger when the user mentions 'Google style', 'gts', 'TypeScript style guide', or asks 'how should I name this', 'should I use interface or type', 'how to handle null/undefined' in a TypeScript context. TypeScriptを書く・レビューする・リファクタリングする、ESLintの設定をする、「命名規則」「型の使い方」「import/exportのルール」「interfaceとtypeどっち」「anyを使っていい？」「nullチェックどうする」「Google式」「コーディング規約」「スタイルガイド」などの質問にも対応します。TypeScriptのコードに関する質問や作業であれば積極的にこのスキルを使ってください。"
---

# Google TypeScript Style Guide Reference

Google TypeScript Style Guide に基づくコーディングリファレンスです。TypeScriptの実装・コードレビュー・リファクタリング・ESLint設定時に、Googleのコーディング規約に準拠するために使います。

## How to use this reference

This guide is organized into topic-specific reference files. Read the relevant file based on the current task:

| Topic | File | When to read |
|-------|------|-------------|
| File structure & imports/exports | `references/source-file-structure.md` | Organizing files, import/export patterns, module structure |
| Language features | `references/language-features.md` | Variables, arrays, objects, classes, functions, control flow, decorators |
| Naming conventions | `references/naming.md` | Naming variables, classes, constants, type parameters, files |
| Type system | `references/type-system.md` | Type inference, null/undefined, any/unknown, interfaces vs types, generics |
| Comments & toolchain | `references/comments-and-toolchain.md` | JSDoc, comments, @ts-ignore rules, conformance |

## Quick Reference: Most Common Rules

These are the rules that come up most frequently. For details, read the corresponding reference file.

### Naming (see `references/naming.md`)
- Classes/Interfaces/Types/Enums: `UpperCamelCase`
- Variables/functions/methods/properties: `lowerCamelCase`
- Global constants & enum values: `CONSTANT_CASE`
- Treat acronyms as words: `loadHttpUrl`, not `loadHTTPURL`
- No `IMyInterface` prefix for interfaces
- No trailing/leading underscores for private members

### Imports & Exports (see `references/source-file-structure.md`)
- Use ES6 module syntax only (`import`/`export`), never `require`
- Use named exports exclusively; avoid default exports
- Use `import type` for type-only imports
- Do not use `namespace`; use modules
- No mutable exports (`export let` is prohibited)

### Variables & Types (see `references/language-features.md`, `references/type-system.md`)
- Use `const` by default, `let` when reassignment needed; never `var`
- Use `unknown` over `any`; avoid `any` wherever possible
- Prefer `interface` over type literal aliases
- Simple arrays: `T[]`; complex element types: `Array<{...}>`
- Prefer optional (`field?`) over `| undefined`

### Functions & Classes (see `references/language-features.md`)
- Prefer function declarations for named functions
- Use arrow functions for callbacks, not function expressions
- Mark never-reassigned properties as `readonly`
- Use parameter properties in constructors
- No `#private` fields; use TypeScript's `private` keyword

### Control Flow (see `references/language-features.md`)
- Always use `===`/`!==` (exception: `== null` for null/undefined check)
- Always use braces for control statements
- Only throw `Error` instances
- All `switch` must have `default` case
- Explicitly end statements with semicolons (no ASI reliance)

### ESLint/Linter Configuration Guide

When configuring ESLint or other linters for Google TypeScript style compliance, enforce these core rules:

**Must enable:**
- `@typescript-eslint/no-explicit-any` - Discourage `any` usage
- `@typescript-eslint/explicit-module-boundary-types` - Encourage explicit return types at module boundaries
- `@typescript-eslint/no-non-null-assertion` - Discourage `!` assertions
- `eqeqeq` with `smart` option - Enforce `===`/`!==` with `== null` exception
- `no-var` - Forbid `var`
- `prefer-const` - Prefer `const` over `let` when no reassignment
- `curly` - Require braces for all control statements
- `@typescript-eslint/no-namespace` - Forbid `namespace`
- `no-eval` - Forbid `eval()`
- `no-debugger` - Forbid `debugger`
- `@typescript-eslint/no-wrapper-object-types` - Forbid `String`, `Boolean`, `Number` wrapper types
- `@typescript-eslint/consistent-type-imports` - Enforce `import type` for type-only imports
- `no-restricted-syntax` for `WithStatement` - Forbid `with`

**Naming convention rules (`@typescript-eslint/naming-convention`):**
- `UpperCamelCase` for classes, interfaces, type aliases, enums, decorators
- `lowerCamelCase` for variables, functions, methods, parameters, properties
- `CONSTANT_CASE` for global constants and enum members
- No leading/trailing underscores

**Import/export rules:**
- `no-restricted-exports` or plugin rules to forbid default exports
- `@typescript-eslint/no-require-imports` - Forbid `require()`

**Additional recommended:**
- `@typescript-eslint/prefer-for-of` - Prefer `for...of`
- `@typescript-eslint/no-unnecessary-type-assertion` - Remove unnecessary assertions
- `@typescript-eslint/prefer-optional-chain` - Prefer optional chaining
- `@typescript-eslint/prefer-readonly` - Prefer `readonly` for never-reassigned members