Exhaustive Discriminated Union (never)

A discriminated union is a union where every member has a shared literal property (the discriminant). TypeScript uses it to narrow types in a switch:

type Shape =
  | { kind: 'circle';    radius: number }
  | { kind: 'rectangle'; width: number; height: number }

function area(shape: Shape): number {
  switch (shape.kind) {
    case 'circle':    return Math.PI * shape.radius ** 2;
    //                                 ^ TypeScript: shape is Circle ✅
    case 'rectangle': return shape.width * shape.height;
    //                          ^ TypeScript: shape is Rectangle ✅
  }
}

Exhaustiveness checking with never

The never type represents impossible values. If you add a new shape variant and forget to handle it, TypeScript catches it via assertNever:

function assertNever(x: never): never {
  throw new Error('Unhandled: ' + JSON.stringify(x));
}

// In the switch default:
default: return assertNever(shape);
// If Triangle is not handled, shape is still Triangle here → TypeScript error:
// Argument of type 'Triangle' is not assignable to parameter of type 'never'

Implement area(shape: Shape)