Check If A Variable Is undefined or null

Check If A Variable Is undefined or null in JavaScript

In this example, you will learn to write a JavaScript program that will check if a variable is undefined or null.

To understand this example, you should have the knowledge of the following JavaScript programming topics:

JavaScript null and undefined

JavaScript typeof Operator

JavaScript Function and Function Expressions

Example 1: Check undefined or null

In the above program, a variable is checked if it is equivalent to null.

The null with == checks for both null and undefined values. This is because null == undefined evaluates to true.

Example 2: using typeof

The typeof operator for undefined value returns undefined. Hence, you can check the undefined value using typeof operator. Also, null values are checked using the === operator.

Note: We cannot use the typeof operator for null as it returns object.