How to check if an object is of a particular class in JavaScript?

Published August 8, 2020

To check if an object or an instance belongs to a certain class you need to use the instanceof operator.

To understand it better let's create a class User and instance of User class called John.

// class User
class User {
  sayHello() {
    console.log("Hi, I'm John");
  }
}

// create an instance or object
// of User class called John.
const John = new User();

Now let's check if the instance John belongs to class User using the instanceof operator.

// class User
class User {
  sayHello() {
    console.log("Hi, I'm John");
  }
}

// create an instance or object
// of User class called John.
const John = new User();

// check if John instance is of class User
console.log(John instanceof User); // true
  • The syntax is objectName instanceOf className.
  • The instanceof operator returns boolean true if it belongs and false if it doesn't belong to the particular class.

You can also use the instanceof operator to check for the instances of normal constructor functions.

// User constructor funstion
function User(name) {
  this.name = name;
}

// instance of User construtor funtion
const John = new User("John Doe");

// check if John instance is of constructor function User
console.log(John instanceof User); // true

One important thing to note with instanceof operator is it will return true for all the classes if it belongs to the prototype chain.

Consider these 2 classes User and Admin, Where Admin class inherits all the properties and methods from the User class.

// class User
class User {
  sayHello() {
    console.log("Hi, I'm John");
  }
}

// class Admin
class Admin extends User {
  givePermissions() {
    console.log("Giving full permissions");
  }
}

// create an instance or object
// of Admin class called John.
const John = new Admin();

// check if John instance is of class User
// the instanceof operator will return true
// since the User class is in the prototype chain
console.log(John instanceof User); // true

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