TypeScript – Check if Class implements an Interface

Unlike classes, interfaces exist only at compile-time, they are not included into the resulting JavaScript, so you cannot do an instanceof check.

You could make IWalkingAnimal a subclass of Animal (and use instanceof), or you could check if the object in question has a walk method:

if (animal['walk']) {}

You can wrap this in a user defined type guard (so that the compiler can narrow the type when used in an if statement, just like with instanceof).

* User Defined Type Guard!
function canWalk(arg: Animal): arg is IWalkingAnimal {
   return (arg as IWalkingAnimal).walk !== undefined;

private moveAnimal(animal: Animal) {
    if (canWalk(animal)) {
        animal.walk();  // compiler knows it can walk now

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