Understanding Undefined in JavaScript

What is Undefined?

In JavaScript, undefined is a primitive value that represents the absence of a value. It is one of the two falsy values in JavaScript, along with null.

Undefined is assigned to variables that have not been assigned a value, or to function parameters that are not passed a value.

How to Check for Undefined

You can check if a variable is undefined using the === operator.


let x;

if (x === undefined) {
// x is undefined
}


When is Undefined Used?

Undefined is often used to indicate that a value is not yet known or has not been assigned.

For example, the following code defines a function that returns the name property of an object, or undefined if the property does not exist.


function getName(obj) {
return obj.name || undefined;
}


Undefined vs. Null

Undefined and null are both falsy values, but they are not the same.

• Undefined represents the absence of a value, while null represents an intentionally assigned empty value.
• Undefined is assigned to variables that have not been assigned a value, while null is explicitly assigned to variables.

Conclusion

Undefined is a primitive value in JavaScript that represents the absence of a value. It is important to understand how undefined is used in JavaScript, and how to check for it.