In JavaScript, understanding object relationships and type checks is crucial for the effective programming. The Two methods that facilitate these checks are instanceof and isPrototypeOf(). While both are used to the determine the type or prototype relationships in the JavaScript they operate differently and are suited for the different scenarios.

These are the following topics that we are going to discuss:

What is instanceof?

The instanceof operator in JavaScript is used to the check if an object belongs to the particular class or constructor function. It returns true if the object is an instance of the specified class or constructor otherwise false.

Characteristics:

• Type Check: Determines if an object is an instance of the specific class or constructor.
• Operator: instanceof
• Usage: object instanceof Class
• Returns: Boolean

Applications:

• Checking object types during the runtime.
• Ensuring compatibility with the expected object prototypes.

Example: This example shows the use of instanceof operator.

class Vehicle { }
class Car extends Vehicle { }
let car = new Car();
console.log(car instanceof Car);
console.log(car instanceof Vehicle);
console.log(car instanceof Object);   

true
true
true

What is isPrototypeOf()?

The isPrototypeOf() method checks if an object exists in the another object’s prototype chain. It returns true if the specified object is in the prototype chain of the object on which it is called otherwise false.

Characteristics

• Prototype Chain Check: Verifies if an object is present in the another object’s prototype chain.
• Method: isPrototypeOf()
• Usage: prototypeObject.isPrototypeOf(object)
• Returns: Boolean

Applications

• The Verifying inheritance relationships.
• Checking if an object inherits properties and methods from the another object.

Example: This example shows the use of isPrototypeOf() method.

function Animal() { }
function Dog() { }
Dog.prototype = Object.create(Animal.prototype);
let dog = new Dog();
console.log(Animal.prototype.isPrototypeOf(dog));
console.log(Object.prototype.isPrototypeOf(dog));
console.log(Dog.prototype.isPrototypeOf(dog));     

true
true
true

Difference Between instanceof and isPrototypeOf()