When every type in a union contains a common property with literal types, TypeScript considers that to be a discriminated union, and can the members of the union.
interface Circle {
kind: "circle"; // common property with literal types
radius: number;
}
interface Square {
kind: "square"; // common property with literal types
sideLength: number;
}
// ⭐️ `kind` is considered a discriminant property of `Shape`
type Shape = Circle | Square;
// switch on shape.kind
function getArea(shape: Shape) {
switch (shape.kind) {
case "circle": return Math.PI * shape.radius ** 2;
case "square": return shape.sideLength ** 2;
}
}