In JavaScript, null and undefined are both special values that represent the absence of a value, but they are different in their meaning and behavior.

undefined is a built-in value in JavaScript that is automatically assigned to a variable when it is declared but not assigned a value. It can also be explicitly assigned to a variable. It is often used to indicate a variable or property that has not been assigned a value yet.

For example, if you declare a variable but do not assign a value to it, its value will be undefined:

let x;

console.log(x); // prints "undefined"

On the other hand, null is a value that represents the intentional absence of any object value. It is often used to indicate that a variable or property has no value, or that a function returns no value.

For example, you can explicitly assign null to a variable:
let y = null;

console.log(y); // prints "null"


One key difference between null and undefined is that null is a primitive value, while undefined is both a primitive value and a type. Additionally, null can be explicitly assigned to a variable, while undefined is usually automatically assigned by JavaScript.

In summary, undefined represents the absence of a value because a variable has not been assigned a value or a property does not exist, while null represents the intentional absence of any object value.