Check if a JavaScript Variable is Undefined

In this tutorial, I will explain to you how you can check if JavaScript Variable is undefined.

Variables in JavaScript

We can declare JavaScript variable using the following four keywords:-

• const
• var
• let
• nothing

Difference between undefined and undeclared variables in JavaScript

Undefined:-  undefined variable is one that has declared but doesn’t have assigned a value. Undefined is not a keyword.

Undeclared:- It occurs when we don’t initialize a variable or declare it using var or const keywords. When we use typeof with undeclared variable it will throw an error with return value as ‘undefined’.

Now to check whether a JavaScript variable is undefined

As here we need to check a variable for undefined:-

JavaScript has a built-in method that is used to check whether a method is defined or undefined.

typeof is a built-in method which check whether a variable is undefined or defined.

typeof doesn’t throw any exception if we declare an undefined variable. We can also use typeof in cheking for null.

var s;
console.log(typeof s); //print undefined

var s1;
console.log(s1===undefined);  //print true
//Since,here the variable s1 don’t have any value but the variable exists.//Here the assigned variables don’t have any value but the variable exisOutput:-

var s3=56;;
console.log(typeof s3); //print type of the value assigned: i.e, Number

Output

Successfully checked if the variable is defined or undefined.

Therefore typeof is the best way to check whether a variable is defined or not in JavaScript.