Discriminated Union Area

Model a Shape as a discriminated union and write area(shape) that returns its area, narrowing on the kind field.

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "rectangle"; width: number; height: number }
  | { kind: "square"; size: number }

Signature: area(shape: Shape) => number

• circle → π r²
• rectangle → width * height
• square → size²

Use a switch (shape.kind) so TypeScript narrows each branch. Add a default that assigns to never for exhaustiveness.