How to use Type Guards in TypeScript ?

Last Updated : 24 Jan, 2025

Here are the methods to use type guards in TypeScript:

1. Using typeof Type Guards

The typeof operator checks the type of a variable, primarily for primitive types like string, number, boolean, etc.

JavaScript

function processValue(value: string | number) {
  if (typeof value === 'string') {
    console.log(value.toUpperCase());
  } else {
    console.log(value.toFixed(2));
  }
}

processValue('Hello');
processValue(123.456);

If value is a string, toUpperCase() is called.
If value is a number, toFixed(2) is called.

Output:

HELLO
123.46

2. Using instanceof Type Guards

The instanceof operator checks if an object is an instance of a specific class or constructor function.

JavaScript

class Dog {
  bark() {
    console.log('Woof!');
  }
}

class Cat {
  meow() {
    console.log('Meow!');
  }
}

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

const myDog = new Dog();
const myCat = new Cat();

makeSound(myDog);
makeSound(myCat);

If animal is an instance of Dog, bark() is called.
If animal is an instance of Cat, meow() is called.

Output:

Woof!
Meow!

3. Using the in Operator

The in operator checks if a property exists in an object.

JavaScript

interface Bird {
  fly: () => void;
}

interface Fish {
  swim: () => void;
}

function move(animal: Bird | Fish) {
  if ('fly' in animal) {
    animal.fly();
  } else {
    animal.swim();
  }
}

const pigeon: Bird = { fly: () => console.log('Flying') };
const salmon: Fish = { swim: () => console.log('Swimming') };

move(pigeon);
move(salmon);

If animal has a fly method, it's a Bird, and fly() is called.
If animal has a swim method, it's a Fish, and swim() is called.

Output:

Flying
Swimming

4. Using User-Defined Type Guards

Custom type guard functions use type predicates to narrow down types.

JavaScript