What is the difference between undefined and is not defined?

In JavaScript, undefined is a value that indicates that a variable or property has not been assigned a value.

On the other hand, "is not defined" is a reference error that occurs because the variable or property is not defined in the current scope, and it cannot be accessed.

Here is an example that illustrates the difference between undefined and is not defined:

let x;
console.log(x); // logs "undefined"

console.log(y); // throws "ReferenceError: y is not defined"