How to Check if the Variable is Undefined in JavaScript

As a developer, you often work with variables in JavaScript.

However, sometimes, the variable you are using may not be defined, which can cause errors in your code.

In this Javascript tutorial, we will explore different ways to check if a variable is undefined in JavaScript.

Why Check for Undefined Variables

Undefined variables can cause unexpected behavior in your code and may lead to bugs and errors.

For example, if you try to access a property of an undefined variable, it will throw a TypeError.

That’s why it’s crucial to check if a variable is undefined before accessing it.

Using the typeof operator

The typeof operator is used to check the type of a variable.

It returns a string that represents the type of the variable.

To check if a variable is undefined, you can use the following code:

let myVariable;
if (typeof myVariable === 'undefined') {
  console.log('The variable is undefined');
} else {
  console.log('The variable is defined');
}

Using the comparison operator

You can also check for undefined variables using the comparison operator.

To do this, you can compare the variable with undefined.

The following code demonstrates this:

let myVariable;
if (myVariable === undefined) {
  console.log('The variable is undefined');
} else {
  console.log('The variable is defined');
}

Using the typeof and comparison operator

You can combine the typeof operator and the comparison operator to check for undefined variables.

This method is considered the safest as it covers all cases of undefined variables, including those that are explicitly set to undefined.

The following code demonstrates this:

let myVariable;
if (typeof myVariable === 'undefined' || myVariable === undefined) {
  console.log('The variable is undefined');
} else {
  console.log('The variable is defined');
}