When defining a set of polymorphic types like Shape, it’s easy to start with:
type Shape = {
kind: 'circle' | 'rect';
radius?: number;
width?: number;
height?: number;
}
function getArea(shape: Shape) {
return shape.kind === 'circle' ?
Math.PI * shape.radius! ** 2
: shape.width! * shape.height!;
}
The non-null assertions (when accessing radius, width, and height fields) are needed because there’s no established relationship between kind and other fields. Instead, discriminated union is a much better solution:
type Circle = { kind: 'circle'; radius: number };
type Rect = { kind: 'rect'; width: number; height: number };
type Shape = Circle | Rect;
function getArea(shape: Shape) {
return shape.kind === 'circle' ?
Math.PI * shape.radius ** 2
: shape.width * shape.height;
}