What is undefined in JavaScript?

In JavaScript, undefined is a primitive value that represents the absence of a value.
It is one of the two special values in JavaScript, the other being null.

When is a variable undefined?

A variable is undefined when it has not been assigned a value.
For example, the following code declares a variable x but does not assign it a value:

“`javascript
let x;

console.log(x); // undefined
“`

A variable can also be undefined if it is assigned the value undefined:

“`javascript
let x = undefined;

console.log(x); // undefined
“`

How to check if a variable is undefined

You can use the typeof operator to check if a variable is undefined:

“`javascript
let x;

console.log(typeof x); // “undefined”
“`

Undefined vs. null

undefined and null are two different values in JavaScript.
undefined represents the absence of a value, while null represents a deliberate “empty” value.

The following table summarizes the key differences between undefined and null:

Property	undefined	null
Value	Represents the absence of a value	Represents a deliberate “empty” value
Type	Primitive value	Object value
Comparison	undefined is not equal to null	null is equal to null

Conclusion

undefined is a primitive value in JavaScript that represents the absence of a value.
It is important to understand the difference between undefined and null, as they are two distinct values with different meanings.