Pattern Matching with Discriminated Unions

              
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type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number }
  | { kind: "rectangle"; width: number; height: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      // Only a circle has a radius property
      return Math.PI * shape.radius * shape.radius;
    case "square":
      // Only a square has a side property
      return shape.side * shape.side;
    case "rectangle":
      // Only a rectangle has width and height properties
      return shape.width * shape.height;
  }
}