TN 7

shape.ts

interface Circle {
  kind: 'circle';
  radius: number;
}

interface Square {
  kind: 'square';
  sideLength: number;
}

type Shape = Circle | Square;

function getArea(shape: Shape) {
  switch (shape.kind) {
    case 'circle':
      // shape는 Circle로 추론됨.
      return shape.radius * shape.radius * Math.PI;
    case 'square':
      // shape는 Square로 추론됨.
      return shape.sideLength * shape.sideLength;
    default:
      throw new Error('Not implemented shape.');
  }
}