Null vs Undefined in JavaScript

In this tutorial, we will learn the basic differences between Null and Undefined in JavaScript.

The difference between NULL and Undefined is as follows:

NULL:

The null keyword in JavaScript specifies that a particular variable or object has not been assigned any value.

var num1 = null;
var num2 = 3;
console.log(num1)
console.log(num2)

The output will be:

null
3

In other words, it can be said that:

  • NULL is an empty non-existent value
  • For any computation, it must be assigned with a value
  • When a variable is assigned to null, its value is null.
  • When a data type is assigned to null, the data type is an object
var a= null;
var b= 3;
var c= a+b;
console.log(c);

Output:

3
var a= null; 
var b= 3; 
a=4;
var c= a+b; 
console.log("sum is: ",c);

Output:

sum is: 7

Undefined:

Undefined is a JavaScript global variable that creates during runtime. It is created when:

  • An object or variable is called but not defined.
  • When array index out of bound is assigned a value
  • When a variable is assigned to undefined, its value is undefined.
  • When a variable is assigned to undefined, the data type is considered undefined
  • Unlike null, undefined is not a reserved keyword in JavaScript

An undefined function parameter is called

var demo;
console.log(demo)

The following code will show error and thus “Undefined” will be called.

Output:

undefined

One can also explicitly set a variable to be undefined as shown in the example below:

var c = undefined;
console.log(c);

The above code will call “undefined”.

Output:

undefined

Also read: How to remove a particular element from Array in JavaScript

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