Object.prototype.isPrototypeOf()

Introduction

The Object.prototype.isPrototypeOf() method is used to determine if an object exists in the prototype chain of another object. This method is available on the Object.prototype object and can be called on any object.

How it works ?

The Object.prototype.isPrototypeOf() method checks whether the prototype property of the specified object exists in the prototype chain of another object. If the prototype property is found in the chain, the method returns true; otherwise, it returns false.

Examples

a. Checking if an object is in the prototype chain of another object:

const animal = { eats: true };
const dog = { barks: true };
const puppy = Object.create(dog);

console.log(animal.isPrototypeOf(puppy)); // false
console.log(dog.isPrototypeOf(puppy)); // true
console.log(Object.prototype.isPrototypeOf(puppy)); // true

In the above example, animal and dog are two objects, and puppy is an object that is created using the Object.create() method with dog as its prototype. We use the Object.prototype.isPrototypeOf() method to check if animal and dog exist in the prototype chain of puppy.

b. Checking if an object is in the prototype chain of a custom object:

function Animal() {}
function Dog() {}
Dog.prototype = Object.create(Animal.prototype);
const puppy = new Dog();

console.log(Animal.prototype.isPrototypeOf(puppy)); // true
console.log(Dog.prototype.isPrototypeOf(puppy)); // true
console.log(Object.prototype.isPrototypeOf(puppy)); // true

Detailed explanation

Syntax

Object.prototype.isPrototypeOf(obj)

The Object.prototype.isPrototypeOf() method is called on an object that exists in the prototype chain of another object, and the method takes a single parameter obj, which is the object whose prototype chain we want to check.

Parameter

obj: The object whose prototype chain is to be checked. This is a mandatory parameter.

Return Value

The Object.prototype.isPrototypeOf() method returns a boolean value. If the prototype property of the specified object exists in the prototype chain of another object, the method returns true; otherwise, it returns false.

Limitations

The Object.prototype.isPrototypeOf() method can only be called on objects. It cannot be called on null or undefined values.

This method only checks if the calling object exists in the prototype chain of the specified object. It does not check if the specified object is an instance of the calling object.

Common use cases:

Some common use cases of Object.prototype.isPrototypeOf() are:

• Checking if an object is an instance of a particular constructor function.
• Checking if an object inherits properties and methods from another object.

Tips and tricks:

• Use Object.prototype.isPrototypeOf() to check if an object exists in another object's prototype chain.
• Do not use Object.prototype.isPrototypeOf() to check if an object is an instance of a constructor function. Use the instanceof operator instead.
• Object.prototype.isPrototypeOf() is similar to the instanceof operator, but the instanceof operator checks if an object is an instance of a constructor function while Object.prototype.isPrototypeOf() checks if an object exists in another object's prototype chain.

Ecmascript specification

See ECMAScript specification

All modern browsers support Object.prototype.isPrototypeOf().

Exercises

1. Create a constructor function Person with properties name and age. Create a new object john with Person constructor and add a method sayHello(). Check if john is an instance of Person using Object.prototype.isPrototypeOf().

function Person(name, age) {
  this.name = name;
  this.age = age;
}

Person.prototype.sayHello = function() {
  console.log("Hello, my name is " + this.name);
};

const john = new Person("John", 30);

console.log(Person.prototype.isPrototypeOf(john)); // true

2. Create a new object obj and assign it a prototype using Object.create(). Check if Object.prototype is in the prototype chain of obj using Object.prototype.isPrototypeOf().

const obj = Object.create({ prop: "value" });

console.log(Object.prototype.isPrototypeOf(obj)); // true

3. Check if an object myObj is an instance of a custom constructor function MyConstructor using Object.prototype.isPrototypeOf().

function MyConstructor() {
  this.prop = "value";
}

const myObj = new MyConstructor();

console.log(MyConstructor.prototype.isPrototypeOf(myObj)); // true