Fake ADTs in TypeScript

adt.ts

type Meters = { meters: number };
type Feet = { feet: number };
type Distance = Meters | Feet;
function isMeters(distance: Distance): distance is Meters {
	return typeof (distance as Meters).meters === 'number';
}
function isFeet(distance: Distance): distance is Feet {
	return typeof (distance as Feet).feet === 'number';
}

function checkDistance(x: Distance): string {
	if (isMeters(x)) {
		return "meters";
	} else if (isFeet(x)) {
		return "feet";
	}
	return "neither";
}