Cast "null" as null in javascript

I use the session storage to set a key that can take numeric values or null. I noticed that the field is stored as a string no matter what I put in. Is there a nice way to convert back my stored value to null ?

For instance, I want to do a check in my code as such:

if (!sessionStorage.getItem('key')) {
   // do something
}

Whats the best way to make that check? The obvious way would be

 if (!sessionStorage.getItem('key') !== "null") {
       // do something
 }

I am wondering if there's an alternative.