Discriminated Unions

interface Circle { kind: "circle"; radius: number; }
interface Square { kind: "square"; sideLength: number; }
interface Rectangle { kind: "rectangle"; width: number; height: number; }

// The Union
type Shape = Circle | Square | Rectangle;

function getArea(shape: Shape) {
  // TypeScript doesn't know what shape it is yet.
  // console.log(shape.radius); // Error!

  switch (shape.kind) {
    case "circle":
      // TS knows 100% it's a circle here
      return Math.PI * shape.radius ** 2;
    case "square":
      // TS knows 100% it's a square here
      return shape.sideLength ** 2;
    case "rectangle":
      return shape.width * shape.height;
    default:
      // EXHAUSTIVENESS CHECKING:
      // If someone adds a 'Triangle' to the Shape union but forgets to 
      // add a case here, TS throws an error because Triangle cannot be assigned to never.
      const _exhaustiveCheck: never = shape;
      return _exhaustiveCheck;
  }
}