Typescript:接口中跨属性的类型保护

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

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

type Shape = Circle | Square;

function calculateArea(shape: Shape): number {
  if (shape.kind === "circle") {
    // 在这里，Typescript会智能地推断出shape是Circle类型
    return Math.PI * shape.radius ** 2;
  } else if (shape.kind === "square") {
    // 在这里，Typescript会智能地推断出shape是Square类型
    return shape.sideLength ** 2;
  }
}

const circle: Circle = { kind: "circle", radius: 5 };
const square: Square = { kind: "square", sideLength: 5 };

console.log(calculateArea(circle)); // 输出78.53981633974483
console.log(calculateArea(square)); // 输出25