switch (true) による絞り込み

[jamashita]

close #1016

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
book	Ready	Preview	Comment	Oct 24, 2025 11:49am

[Copilot]

Pull Request Overview

This PR adds documentation for using switch (true) for type narrowing in TypeScript and removes an outdated note about TypeScript version requirements for type guard variable assignment.

• Adds comprehensive section explaining switch (true) pattern for type narrowing
• Removes note about TypeScript 4.4 version requirement for variable type guards
• Includes examples with typeof, instanceof, and user-defined type guard functions

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The type guard functions always return true which is misleading for documentation purposes. These should contain actual type checking logic (e.g., checking for the existence of properties like 'panda' in value) to demonstrate proper implementation of user-defined type guards.

Suggested change

	return true;
	};
	const isBroccoli = (value: unknown): value is Broccoli => {
	return true;
	};
	const isUser = (value: unknown): value is User => {
	return true;
	return typeof value === "object" && value !== null && "panda" in value;
	};
	const isBroccoli = (value: unknown): value is Broccoli => {
	return typeof value === "object" && value !== null && "broccoli" in value;
	};
	const isUser = (value: unknown): value is User => {
	return typeof value === "object" && value !== null && "name" in value;