Exhaustiveness Checking with the Assert never Technique

function area(shape: Shape) {
  switch (shape.kind) {
    case "circle": // shape is Circle
      return Math.PI * shape.radius * shape.radius;
    case "triangle": // shape is Triangle
      return (shape.x * shape.y) / 2;
    case "square": // shape is Square
      return shape.x * shape.x;
    case "rectangle":
      return shape.x * shape.y;
    default: // shape is never
      assertNever(shape); // shape can be passed to assertNever!
  }
}