Type Guards vs. Conditional Types

guard-vs-cond.ts

type numeric = number | bigint

const num_guard = (x: unknown): x is numeric =>
	typeof x == 'number' || typeof x == 'bigint'

const num_cond = <T,>(x: T) => (
	typeof x == 'number' || typeof x == 'bigint'
) as T extends numeric ? true : false

// this example is incomplete,
// I just kept it here because of the type-assertions
const f = <T,>(x: T): T extends numeric ? T : undefined => {
	if (num_guard(x))
		return x as T extends numeric ? T : never
	return undefined as T extends numeric ? never : undefined
}