The Never Type

Never Type

👨‍💼 Some functions never return. Not because they return void (nothing), but because they can't return—they throw an error or run forever.

function fail(message: string): never {
	throw new Error(message)
}

TypeScript uses this to mark code paths that can't continue. A never function is one that always throws or exits early.

🐨 Open

index.ts

and:

Create a throwError function that throws and returns never
Create a parseNumber function that converts a string to a number, using throwError("Invalid number") if the input can't be parsed
Create an ensurePositive function that returns the number if positive, or uses throwError("Number must be positive") if negative

The key learning here is that parseNumber and ensurePositive should use the throwError function rather than throwing directly. This demonstrates how TypeScript's flow analysis understands that code after a never-returning function call is unreachable.

Number(value) converts a string to a number. If the string can't be parsed, it returns NaN (Not a Number). Check for NaN using Number.isNaN(result).

📜 TypeScript Handbook - never

06. Void and Never/02. The Never Type (💪 problem)

The Never Type