Union types and narrowing

string | number means "either a string or a number" - a union of two types. A parameter typed id: string | number could genuinely be handed either one, so before doing anything type-specific with it, you need to check which type you actually got. That check is called a type guard, and the simplest one is typeof:

if (typeof id === 'number') {
  // id is treated as a number here
}

Inside that if, the value is narrowed to just number - outside it, or in an else, it's still the other option. This is the pattern that makes a union type useful rather than just a wider hole to fall through.

Your task: write formatId(id) - if id is already a string, return it unchanged; if it's a number, return it as a string prefixed with #.

You'll practice:

Declaring a union type with |
Narrowing a union with a typeof check

// formatId returns a string id unchanged, but prefixes a numeric id with "#".
function formatId(id: string | number): string {

}

const a = formatId("abc123");
const b = formatId(42);