How to Check If a Variable is “undefined” in JavaScript

To check if a variable is “undefined” in JavaScript, use the “strict equality operator(===)” or “typeof operator” along with the “strict equality (===)” operator.

Method 1: Using the strict equality operator(===)

In JavaScript, the easy way to check if a variable is “undefined” is by using the “strict equality operator”(===).

Example

let myVar;

if (myVar === undefined) {
  console.log('myVar is undefined');
} else {
  console.log('myVar is defined');
}

Output

myVar is undefined

In this example, myVar is declared but not assigned a value, so its value is undefined.

Method 2: Using the typeof operator with === operator

You can use the typeof operator in JavaScript to check if a variable is undefined. The typeof operator returns a string suggesting the type of the operand.

Example

let data = undefined;

if (typeof (data) === undefined) {
  console.log('Yes in fact it is undefined');
}

Output

Yes in fact it is undefined

The typeof operator “returns the string, indicating a type of unevaluated operand”.  The operator returns a data type of its operand as a string.

That’s it.

Krunal

Krunal Lathiya is a seasoned Computer Science expert with over eight years in the tech industry. He boasts deep knowledge in Data Science and Machine Learning. Versed in Python, JavaScript, PHP, R, and Golang. Skilled in frameworks like Angular and React and platforms such as Node.js. His expertise spans both front-end and back-end development. His proficiency in the Python language stands as a testament to his versatility and commitment to the craft.