Let’s talk about the similarities first.

null and undefined are JavaScript primitive types.

The meaning of undefined is to say that a variable has declared, but it has no value assigned.

let age //age is undefined

let age = null //age is null

Note: accessing a variable that’s not been declared will raise a ReferenceError: <variable> is not defined error, but this does not mean it’s undefined.

How do you check if a variable is null? Use the comparison operator, for example age === null

Same for undefined: age === undefined

In both cases, you can check for:

if (!age) {

}

and this will be matching both null and undefined.

You can also use the typeof operator:

let age
typeof age //'undefined'

although null is evaluated as an object, even though it is a primitive type:

let age = null
typeof age //'object'