箭头样式用户定义的类型保护？

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

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

type Shape = Square | Circle;

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

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

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

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

type Shape = Square | Circle;

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

function getArea(shape: Shape): number {
    if (isSquare(shape)) {
        return shape.size * shape.size;
    } else {
        return Math.PI * shape.radius * shape.radius;
    }
}

const square: Shape = { kind: 'square', size: 10 };
const circle: Shape = { kind: 'circle', radius: 5 };

console.log(getArea(square)); // 输出 100
console.log(getArea(circle)); // 输出 78.53981633974483