class A { 
    name: string;
}
class B { 
    age: number;
}

let x: A | B;

if (x instanceof A) {
    console.log(x.name); // ok: x has type A

    setTimeout(() => {
        console.log(x.name); // ** COMPILE ERROR: x has type A | B here
    });
} else {
    console.log(x.age); // ok: x has type B here

    setTimeout(() => {
        console.log(x.age); // ** COMPILE ERROR: x has type A | B here
    });
}