instanceof requires that the variable is of type any.

This code (try it):

class Pet { }
class Dog extends Pet {
    bark() {
        console.log("woof");
    }
}
class Cat extends Pet {
    purr() {
        console.log("meow");
    }
}

function example(foo: any) {
    if (foo instanceof Dog) {
        // foo is type Dog in this block
        foo.bark();
    }

    if (foo instanceof Cat) {
        // foo is type Cat in this block
        foo.purr();
    }
}

example(new Dog());
example(new Cat());

prints

woof
meom

to the console.