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:

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:

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

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

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.

 

 

 

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.

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