---
title: "what is undefined variable?"  
description: "what is undefined variable?"  
author: "Amit Singh"  
published: 2010-12-31  
updated: 2023-05-03  
canonical: https://www.mindstick.com/interview/309/what-is-undefined-variable  
category: "javascript"  
tags: ["javascript"]  
reading_time: 2 minutes  

---

# what is undefined variable?

Variables which are declared but not assigned any value is called undefined variable.

## Answers

### Answer by Aryan Kumar

In JavaScript, an undefined variable is a variable that has been declared but has not been assigned a value. When a variable is declared but not initialized with a value, it is automatically assigned the value of **undefined**.

For example, consider the following code:

```javascript
var myVar;
console.log(myVar); // Outputs: undefined
```

In this example, **myVar** is declared but not initialized with a value, so it has the value of **undefined**.

In some cases, the use of undefined variables can result in errors or unexpected behavior in your code. To avoid this, it is important to always initialize your variables with appropriate values before using them.

One way to check whether a variable is undefined is to use the **typeof** operator. For example:

```javascript
var myVar;
console.log(typeof myVar); // Outputs: "undefined"
```

You can also use an **if** statement to check if a variable is undefined. For example:

```javascript
var myVar;
if (typeof myVar === "undefined") {
 // do something if myVar is undefined
}
```

In modern JavaScript, it is generally considered better practice to use the **let** and **const** keywords instead of **var**, as they have more predictable behavior and scope. Variables declared with **let** or **const** are automatically initialized with the value of **undefined** if they are not explicitly assigned a value.

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### Answer by Anonymous User

A variable (structure that holds data) that is not available at runtime to the application. This typically occurs with programming languages such as scripts, which allow variables to be located at runtime rather than being resolved ahead of time in a compile stage.\

### Answer by Amit Singh

Variables which are declared but not assigned any value is called undefined variable.


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