What is the difference between null and undefined?

In JavaScript, null and undefined are two different types that both denote the absence of a value but are used in different contexts.

• undefined is the default value for variables that have been declared but not initialized, as well as for missing object properties or function arguments.
• null is an explicit assignment of a value, meaning "no value" or "empty value." It is usually used by programmers to indicate the intentional absence of an object.

Example:

let a;
console.log(a); // undefined — variable declared but not initialized

let b = null;
console.log(b); // null — variable explicitly assigned an empty value

let obj = {};
console.log(obj.prop); // undefined — property does not exist in the object

Thus, undefined is a default state, while null is an explicit indication of the absence of a value.