Null vs Undefined in JavaScript

Introduction

In this tutorial, you will learn about JavaScript Null vs Undefined.

Null and undefined represent empty or missing values.

Many Developers get confused between these two, so I will clear each.

What is null in JavaScript?

null is an intentional empty value. If you declare a variable and define the value null in it.


let name = null;
console.log(name); // null

Note: If you declare a null value, that means it does not have any value.

What is undefined in JavaScript?

undefined means a variable has been declared but not assigned any value.


let name;
console.log(name); // undefined

Difference between Null vs Undefined

You can clear the difference through the details below.

null	undefined
It is a Intentional empty value.	Variable not assigned.
Type of Null is a object.	Type of Undefined is undefined.
Null is declared by Developer.	Undefined is declared by JavaScript.
Null means value exists.	Undefined means value not exists.

Type Checking

Now, you will check Type Checking through below code.

1. typeof null returns Object.

2. typeof undefined returns undefined.


typeof null;      // "object"
typeof undefined; // "undefined"

Comparison of null vs undefined


null == undefined   // true
null === undefined  // false

Note:

== is used to check only the value.

=== is used to check value and type.

Real Life Example: Null vs Undefined in JavaScript

1. Undefined Example (Value Not Assigned Yet)

Suppose, you create a user profile form, but the user has not filled the mobileNumber yet.


let mobileNumber;
console.log(mobileNumber); // undefined

Explanation:

The variable mobileNumber is declared but not assigned a value, so JavaScript automatically sets it to undefined.

2. Null Example (Value Intentionally Empty)

Now suppose the user chooses not to provide a middle name.

Example:


let middleName = null;

console.log(middleName); // null

Explanation:

Here, the developer intentionally assigns null to show that the value does not exist.

Interview Questions

Q 1: What is undefined in JavaScript?

Ans: Undefined means a variable has been declared but no value has been assigned yet.

Q 2: What is null in JavaScript?

Ans: Null is an intentional assignment representing no value.

Q 3: What will this code output console.log(null == undefined);

Ans: true

Because loose equality (==) treats them as equal.

Q 4: What will this code output console.log(null === undefined);

Ans: false

Because strict equality (===) checks both type and value.

Q 5: When should we use null?

Ans: Use null when you intentionally want to indicate no value.

Example:

Reset object

Clear data

No result found

Conclusion

Both null and undefined represent that a value does not exist in JavaScript, but they are used in different situations.

Undefined means a variable has been declared but has not been assigned a value yet.
Null means the developer intentionally sets a variable to an empty value.

If developers understand the difference between null and undefined, then they write cleaner code and avoid common bugs in JavaScript applications.