instanceof type guards in Typescript

Introduction

We use instanceof type guards to check what kind of object we have. This helps us use the right properties or methods safely.

When you have a variable that could be one of several object types.

When you want to run different code depending on the object's class.

When you want to avoid errors by checking the object's type before using it.

When working with class instances and need to confirm their type.

When you want clearer and safer code by telling TypeScript the exact type.

Syntax

Typescript

if (variable instanceof ClassName) {
  // code for ClassName objects
}

instanceof checks if the object was created from the given class or its child classes.

This works only with classes, not interfaces or types.

Examples

This checks if pet is a Dog and then calls bark().

Typescript

if (pet instanceof Dog) {
  pet.bark();
}

This runs drive() if vehicle is a Car, otherwise it runs fly().

Typescript

if (vehicle instanceof Car) {
  vehicle.drive();
} else {
  vehicle.fly();
}

Sample Program

This program defines two classes, Cat and Dog. The speak function uses instanceof to check the pet's type and calls the correct sound method.

Typescript

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

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

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

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

speak(myCat);
speak(myDog);

Important Notes

instanceof only works with objects created from classes, not plain objects or interfaces.

It helps TypeScript know the exact type inside the if block, so you get better code suggestions and safety.

Summary

instanceof checks an object's class to guide safe code.

Use it when you have multiple possible object types.

It works only with classes, not interfaces or types.

Practice

1. What does the instanceof operator check in TypeScript?

easy

A. If a variable is a primitive type

B. If an object is an instance of a specific class

C. If a function returns a boolean

D. If a type is an interface

Solution

1. Step 1: Understand the purpose of instanceof

   The instanceof operator checks whether an object was created from a particular class.

2. Step 2: Differentiate from other checks

   It does not check primitive types, function returns, or interfaces, only class instances.

3. Final Answer:

   If an object is an instance of a specific class -> Option B

4. Quick Check:

   instanceof checks class instance = A [OK]

Hint: Remember: instanceof works only with classes, not interfaces [OK]

Common Mistakes:

• Thinking instanceof works with interfaces
• Confusing instanceof with typeof
• Assuming instanceof checks primitive types

2. Which of the following is the correct syntax to check if obj is an instance of class Car?

easy

A. if (obj instanceof Car) { }

B. if (obj instance Car) { }

C. if (obj is Car) { }

D. if (obj type Car) { }

Solution

1. Step 1: Recall correct instanceof syntax

   The correct syntax uses the keyword instanceof between the object and the class name.

2. Step 2: Identify invalid options

   Options B, C, and D use incorrect keywords or missing syntax.

3. Final Answer:

   if (obj instanceof Car) { } -> Option A

4. Quick Check:

   Correct syntax uses instanceof keyword = A [OK]

Hint: Use 'instanceof' keyword exactly between object and class [OK]

Common Mistakes:

• Using 'is' or 'type' instead of 'instanceof'
• Omitting parentheses around condition
• Writing 'instance' instead of 'instanceof'

3. What will be the output of this code?

class Dog { bark() { return 'woof'; } }
class Cat { meow() { return 'meow'; } }
function speak(animal: Dog | Cat) {
  if (animal instanceof Dog) {
    return animal.bark();
  } else {
    return animal.meow();
  }
}
console.log(speak(new Cat()));

medium

A. 'woof'

B. TypeError at runtime

C. 'meow'

D. undefined

Solution

1. Step 1: Identify the object passed

   The function speak is called with new Cat(), so animal is a Cat instance.

2. Step 2: Check the instanceof condition

   The condition animal instanceof Dog is false, so the else block runs, returning animal.meow().

3. Final Answer:

   'meow' -> Option C

4. Quick Check:

   Cat instance triggers else block = D [OK]

Hint: Trace instanceof condition to pick correct method call [OK]

Common Mistakes:

• Assuming instanceof works with interfaces
• Confusing which method runs for which class
• Expecting 'woof' when passing Cat instance

4. Identify the error in this code snippet:

interface Bird { fly(): void; }
class Eagle implements Bird { fly() { console.log('Flying'); } }
function check(bird: Bird) {
  if (bird instanceof Eagle) {
    bird.fly();
  }
}

medium

A. No error, code is correct

B. Missing return type in fly() method

C. Cannot use instanceof with interfaces

D. Eagle class must be abstract

Solution

1. Step 1: Understand instanceof usage

   The instanceof operator is used with the class Eagle, which is valid even though bird is typed as the interface Bird.

2. Step 2: Verify no errors

   There are no syntax errors, type mismatches, or runtime issues. TypeScript compiles this code and narrows bird to Eagle inside the if block.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   instanceof with class on interface-typed param works = D [OK]

Hint: Remember: instanceof works with classes even on interface-typed variables [OK]

Common Mistakes:

• Thinking instanceof can't be used on interface-typed variables
• Confusing instanceof with direct interface checks
• Thinking implementing class must be abstract

5. Given these classes:

class Animal { name: string; constructor(name: string) { this.name = name; } }
class Fish extends Animal { swim() { return 'swimming'; } }
class Bird extends Animal { fly() { return 'flying'; } }

Write a function move that takes animal: Animal and returns 'swimming' if it's a Fish, 'flying' if it's a Bird, or 'unknown' otherwise. Which implementation is correct?

hard

A. function move(animal: Animal) { if (animal instanceof Animal) return 'unknown'; else if (animal instanceof Fish) return animal.swim(); else return animal.fly(); }

B. function move(animal: Animal) { if (animal instanceof Fish) return 'flying'; else if (animal instanceof Bird) return 'swimming'; else return 'unknown'; }

C. function move(animal: Animal) { if (animal instanceof Fish) return animal.fly(); else if (animal instanceof Bird) return animal.swim(); else return 'unknown'; }

D. function move(animal: Animal) { if (animal instanceof Fish) return animal.swim(); else if (animal instanceof Bird) return animal.fly(); else return 'unknown'; }

Solution

1. Step 1: Check instanceof order and method calls

   The function must check if animal is a Fish or Bird using instanceof and call the correct method.

2. Step 2: Validate each option

   function move(animal: Animal) { if (animal instanceof Fish) return animal.swim(); else if (animal instanceof Bird) return animal.fly(); else return 'unknown'; } correctly checks Fish first, calls swim(), then Bird with fly(), else returns 'unknown'. function move(animal: Animal) { if (animal instanceof Animal) return 'unknown'; else if (animal instanceof Fish) return animal.swim(); else return animal.fly(); } wrongly checks Animal first, which always matches. function move(animal: Animal) { if (animal instanceof Fish) return animal.fly(); else if (animal instanceof Bird) return animal.swim(); else return 'unknown'; } swaps methods incorrectly. function move(animal: Animal) { if (animal instanceof Fish) return 'flying'; else if (animal instanceof Bird) return 'swimming'; else return 'unknown'; } returns incorrect strings directly without calling methods.

3. Final Answer:

   function move(animal: Animal) { if (animal instanceof Fish) return animal.swim(); else if (animal instanceof Bird) return animal.fly(); else return 'unknown'; } -> Option D

4. Quick Check:

   Correct instanceof checks and method calls = B [OK]

Hint: Check instanceof order and call matching methods [OK]

Common Mistakes:

• Checking base class first, blocking subclass checks
• Calling wrong methods for classes
• Returning strings instead of calling methods