Question 48

Question

Explain the difference between instanceof and checking for constructor property in JavaScript.

Answer

1. instanceof:

• Purpose: Checks if an object is an instance of a particular class (or prototype chain). It looks at the object's internal [[Prototype]] chain and verifies if it leads back to the specified constructor function.

• Syntax: object instanceof Constructor (returns true or false)

• Advantages:

  • Intuitive: Directly checks for instance relationship.

  • Efficient: Optimized by JavaScript engines.

• Limitations:

  • Works best with objects created using new.

  • Not reliable for custom classes not built with new.

2. Checking the constructor Property:

• Purpose: Accesses the constructor function associated with an object.

• Syntax: object.constructor === Constructor (returns true or false)

• Advantages:

  • More versatile: Can work with objects created using various methods, including manual assignments.

• Limitations:

  • Can be misleading if the constructor property has been modified manually (e.g., object tampering).

Example:

function Animal(name) {
  this.name = name;
}

const dog = new Animal("Buddy"); // Created using 'new'

console.log(dog instanceof Animal); // true 
console.log(dog.constructor === Animal); // true 

const cat = {}; 
cat.constructor = Animal;  // Manually set the constructor

console.log(cat instanceof Animal); // false 
console.log(cat.constructor === Animal); // true 

Which to Choose:

• For objects created with new, use instanceof. It's generally more reliable and efficient.

• For more flexible checks or when dealing with objects that might not have been created using new, consider checking the constructor property, but be aware of potential manipulation risks.