Explain the differences between null and undefined in programming language.

In JavaScript (and Node.js), null and undefined are two different types that are often confused, but they have different meanings:

• undefined is the default value for variables that have been declared but not initialized. Also, a function without an explicit return statement returns undefined.
• null is an explicit assignment of a value, meaning "no value" or "empty value".

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

function foo() {}
console.log(foo()); // undefined — function does not return anything

The key difference is that undefined usually means that a value has not been assigned, whereas null is an intentional indication of the absence of a value.