TypeScript · Narrowing and unions

in operator

Narrows by checking whether a property exists on the value.

Syntax

if ("id" in value)

Example

type A = { id: string };
type B = { name: string };
function label(value: A | B) {
  return "id" in value ? value.id : value.name;
}

Reference pages are the quick answer. The course explains the why, shows the full example and ends with a challenge.