What is the difference between null and undefined in JavaScript?

In JavaScript, null and undefined are both primitive values that represent the absence of a value or object. However, they have different meanings and uses.

null is a value that represents the intentional absence of an object value. It is an explicit representation of no value. You can assign the value null to a variable as a way of indicating that the variable does not currently have a valid value.

let x = null;

undefined is a value that is assigned to a variable when the variable is declared, but no value is assigned to it. It represents the absence of a value, but it is not the same as null. undefined indicates that the variable has been declared, but it has not been assigned a value.

let y;
console.log(y); // Output: undefined

You can also use the typeof operator to check the type of a value. The typeof operator will return "undefined" for variables that have been declared, but not assigned a value, and "null" for variables that have been assigned the value null.

console.log(typeof x); // Output: "object"
console.log(typeof y); // Output: "undefined"

It’s important to note that, in JavaScript, null is considered an object, while undefined is considered a primitive value. This can be confusing, as typeof null returns "object", while typeof undefined returns "undefined". This is an error in the design of the language and cannot be fixed without breaking backward compatibility.