在typescript中仅使用原始类型属性的一部分创建子类型

type Person = {
  name: string;
  age: number;
};

type Employee = Person & {
  empId: number;
};

const emp: Employee = {
  name: 'John',
  age: 30,
  empId: 12345,
};

type Shape = {
  type: 'circle' | 'rectangle';
  radius?: number;
  width?: number;
  height?: number;
};

type Circle = Shape & {
  type: 'circle';
  radius: number;
};

type Rectangle = Shape & {
  type: 'rectangle';
  width: number;
  height: number;
};

function getArea(shape: Shape): number {
  if (shape.type === 'circle') {
    return Math.PI * shape.radius ** 2;
  } else {
    return shape.width * shape.height;
  }
}

const circle: Circle = {
  type: 'circle',
  radius: 5,
};

const rectangle: Rectangle = {
  type: 'rectangle',
  width: 10,
  height: 20,
};

console.log(getArea(circle)); // 输出：78.53981633974483
console.log(getArea(rectangle)); // 输出：200