Difference Between in and hasOwnProperty in JavaScript

In JavaScript, working with the objects often involves checking for the presence of properties. The two commonly used methods for this purpose are in and hasOwnProperty. Both serve to check for the properties in objects but they have key differences in how they operate and their intended use cases.

These are the following topics that we are going to discuss in this article:

What is in?

The in operator checks whether a specified property exists in an object or its prototype chain. It returns true if the property is found either directly on the object or in its prototype chain and false otherwise.

Characteristics

• Prototype Chain: The in checks not only the object's own properties but also properties in its prototype chain.
• Boolean Return: The Returns a Boolean value indicating the presence of the property.
• Non-method: It is an operator, not a method, so it does not require parentheses.

Applications

• Prototype Inheritance: Useful when we need to check for the properties that might be inherited from the prototype.
• General Existence Check: The Suitable for the general checks where prototype properties are also relevant.

Example: This example shows the use of in operator.

let person = { name: "kumar" };
console.log("name" in person); 
console.log("toString" in person); 
console.log("age" in person); 

true
true
false

What is hasOwnProperty?

The hasOwnProperty method checks whether a specified the property exists directly on an object and not on its prototype chain. It returns true if the property is found directly on the object and false otherwise.

Characteristics

• Direct Properties Only: Checks only the object's own properties ignoring the prototype chain.
• Method Call: It is a method and needs to be called on the object with the parentheses.
• Boolean Return: The Returns a boolean value indicating the presence of the property.

Applications

• Object Specific Checks: Ideal for verifying if a property is an object's own property and not inherited.
• Safeguard Against Prototypes: Useful in scenarios where prototype properties should not be considered.

Example: This example shows the use of hasOwnProperty method.

let person = { name: "kumar" };
console.log(person.hasOwnProperty("name")); 
console.log(person.hasOwnProperty("toString"));
console.log(person.hasOwnProperty("age")); 

true
false
false

Difference Between in and hasOwnProperty

Conclusion

The in operator and hasOwnProperty method are both essential tools for the property checking in JavaScript. Understanding their differences is crucial for their correct application especially in scenarios involving the prototype inheritance. While in is useful for broader checks including the prototypes hasOwnProperty provides a more focused approach by only considering an object's own properties.