Advanced type guards.

FishOrBird.ts

interface Bird {
    fly();
    layEggs();
}

interface Fish {
    swim();
    layEggs();
}

function isFish(pet: Fish | Bird): pet is Fish {
    return (<Fish>pet).swim !== undefined;
}

...
// Both calls to 'swim' and 'fly' are now okay.
if (isFish(pet)) {
    pet.swim();
}
else {
    pet.fly();
}