The other answers are right, but when you're dealing with interfaces you cannot use typeof or instanceof because interfaces don't get compiled to javascript.
Instead you can use a typecast + function check typeguard to check your variable:
interface Car {
drive(): void;
honkTheHorn(): void;
}
interface Bike {
drive(): void;
ringTheBell(): void;
}
function start(vehicle: Bike | Car ) {
vehicle.drive();
// typecast and check if the function exists
if ((<Bike>vehicle).ringTheBell) {
const bike = (<Bike>vehicle);
bike.ringTheBell();
} else {
const car = (<Car>vehicle);
car.honkTheHorn();
}
}
And this is the compiled JavaScript in ES2017:
function start(vehicle) {
vehicle.drive();
if (vehicle.ringTheBell) {
const bike = vehicle;
bike.ringTheBell();
}
else {
const car = vehicle;
car.honkTheHorn();
}
}