type Shape = { kind: 'circle'; r: number } | { kind: 'rect'; w: number; h: number }
switch (shape.kind) {
case 'circle': return Math.PI * shape.r ** 2
case 'rect': return shape.w * shape.h
}A discriminated union uses a shared literal property to select the correct variant.
A discriminated union uses a shared literal property (the discriminant) to distinguish union members.
Challenge: Implement processShape(shape) that calculates area for circles and rectangles.
processShape({ kind: 'circle', radius: 1 }) // → Math.PI
processShape({ kind: 'rect', width: 3, height: 4 }) // → 12