type Shape = { kind: 'circle'; r: number } | { kind: 'square'; s: number }
function isCircle(sh: Shape): sh is Extract<Shape, { kind: 'circle' }> {
return sh.kind === 'circle'
}Checking the discriminant property narrows a union to a specific member type.
A discriminated union uses a shared literal kind field to distinguish variants.
Use a regular switch or if-check on the literal to narrow the type.
Challenge: Implement getArea(shape: Shape): number using a switch on shape.kind.
getArea({ kind: 'circle', radius: 5 }) // → Math.PI * 25
getArea({ kind: 'rectangle', width: 4, height: 3 }) // → 12