hoge.tsx

class X {
    id: number = 3
}

class Y {
    id: number = 9
}

var x = new X()

var y = new Y()

function isX(obj: any): obj is X {
    return obj instanceof X
}

function isY(obj: any): obj is Y {
    return obj instanceof Y
}

function hoge(obj: { id: number }): string {
    // switchは使えない
    if (isX(obj)) {
        return `X: ${obj.id}`
    } else if (isY(obj)) {
        return `Y: ${obj.id}`
    } else {
        return `?: ${obj.id}`
    }
}

console.log()
console.log()

alert(`${hoge(x)}, ${hoge(y)}`)