Difference between undefined and null

NULL is a special value which represents nothing or empty. You can initially set a variable to NULL. For example:

let fname = null;

Maybe we don’t know the name yet. Later we can change it.

fname = "John";

The variable below is undefined:

let age;

it is undefined because a value hasn’t been assigned to it yet.

Null is an assignment value, it is assigned to a variable, as seen above.

We have an article about Javascript data types, check it out.