Andrei C. - 2020-06-20

Undefined vs. null

This is a very interesting today I learned about JavaScript.

Let’s say you have a function friendEmail:

class User {
  get friendEmail() {
    const { sharedEmail } = this.data;

    return sharedEmail ? this.data.email : undefined;
  }
}

Reviewing this, as I did, you might leave a comment saying:

Should we return null here instead of undefined?

And you would we wrong! As per TC39 documentation about null¹ you can see the following:

4.4.15 null value

primitive value that represents the intentional absence of any object value

So a function that returns a primitive² should in the abstinence of value return an undefined and a function that returns an object type should return null.

Keep in mind that returning a null wont however be switching return types, since null it self is a primitive but it only represents absence of any object value.

Weird stuff, right?