instanceof

Learn class narrowing with instanceof.

instanceof

The instanceof operator checks whether an object is an instance of a specific class (or its superclass). An if (animal instanceof Cat) condition checks the prototype chain and lets TypeScript narrow the type to Cat. It is used for narrowing in class-based hierarchies.

class Cat { meow() {} }
class Dog { bark() {} }

function speak(animal: Cat | Dog) {
  if (animal instanceof Cat) {
    animal.meow();  // Cat
  } else {
    animal.bark();  // Dog
  }
}

Class hierarchy

instanceof works only with classes; it cannot be used with interfaces because interfaces have no runtime representation. In a class hierarchy, instanceof treats subclass instances as instances of the superclass too.

class Animal {}
class Cat extends Animal { meow() {} }

const c: Animal = new Cat();
if (c instanceof Cat) {
  c.meow();  // Cat
}
// Does not work with interfaces:
// if (c instanceof Pet) ...  // error if Pet is an interface

Exercise Separate classes with instanceof

Define Cat (with a meow method) and Dog (with a bark method) classes. Write a speak(animal: Cat | Dog): string function. If animal instanceof Cat, return "meow"; otherwise return "bark". Print results via console.log(speak(new Cat())) and console.log(speak(new Dog())).

Show example solution

class Cat {
  meow() {}
}

class Dog {
  bark() {}
}

function speak(animal: Cat | Dog): string {
  if (animal instanceof Cat) {
    return "meow";
  }
  return "bark";
}

console.log(speak(new Cat()));
console.log(speak(new Dog()));

Test yourself

What does the instanceof operator check?
- The equality of two values
- The presence of a property
- The runtime type string of a value
- The prototype chain of an object
Show answer

The prototype chain of an object — instanceof checks an object's prototype chain and returns whether it is an instance of a specific class; TypeScript uses this to narrow the type.

Class check with instanceof

class Cat { meow() {} }
class Dog { bark() {} }

function speak(a: Cat | Dog) {
  if (____) {
    a.meow();
  } else {
    a.bark();
  }
}

Show answer

class Cat { meow() {} }
class Dog { bark() {} }

function speak(a: Cat | Dog) {
  if (a instanceof Cat) {
    a.meow();
  } else {
    a.bark();
  }
}

instanceof parts

instanceof

checks prototype chain
instanceof Cat

narrows to Cat class
Sınıf (class)

can be used with instanceof
Interface

cannot be used with instanceof

Next lesson: Summary - Type narrowing

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