Typescript联合类型到对象类型

type Shape = Square | Circle;

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

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

function area(s: Shape) {
    if (s.kind === "square") {
        // 类型断言
        const sq = s as Square;
        return sq.size * sq.size;
    } else {
        // 类型断言
        const c = s as Circle;
        return Math.PI * c.radius ** 2;
    }
}

function isSquare(shape: Shape): shape is Square {
    return shape.kind === "square";
}

function area(s: Shape) {
    if (isSquare(s)) {
        // 在这个if块内，TypeScript知道s是Square类型
        return s.size * s.size;
    } else {
        // 在这个else块内，TypeScript知道s是Circle类型
        return Math.PI * s.radius ** 2;
    }
}