Type Guards

👨‍💼 We keep writing the same checks to narrow our union types. Let's extract those checks into reusable type guard functions.

In functional programming, these are called predicates—functions that test something and return true or false.

TypeScript type guards use the special is return type to tell the compiler what type a value is when the predicate returns true:

type Cat = { meow(): void }
type Dog = { bark(): void }
type Pet = Cat | Dog

function isCat(pet: Pet): pet is Cat {
	return 'meow' in pet
}

function speak(pet: Pet) {
	if (isCat(pet)) {
		pet.meow() // ✅ TypeScript knows it's a Cat
	} else {
		pet.bark() // ✅ TypeScript knows it's a Dog
	}
}

🐨 Open

index.ts

and:

Create an isStringArray type guard and use it inside normalizeText
Create isAdminUser, isRegularUser, and isGuestUser type guards and use them in describeUser
Export isStringArray, normalizeText, isAdminUser, isRegularUser, isGuestUser, and describeUser by name

Completion criteria

Named exports listed above
isStringArray is true for string arrays and false for strings / number arrays
User guards are true only for their own shape
normalizeText / describeUser keep the same exact outputs as the Narrowing step (for example Admin with 2 permissions, Regular user (free), Guest user)

📜 TypeScript Handbook - Type Guards

02. Union Types/03. Type Guards (💪 problem)

Type Guards

Completion criteria