What is the difference between null and undefined?

In JavaScript, null and undefined are different types that both denote the absence of a value but with different meanings:

• undefined means that a variable has been declared but not assigned a value. This is the default state for undeclared properties or variables.

• null is an explicit assignment of a value, meaning "no value" or "empty value".

Example:

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

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

Thus, undefined is the default absence of a value, while null is an intentional indication of no value.