Check if Email exists before adding to database in Angular and .Net API

Hi,

I have the following service:

this.housingService.subscribeEmail(localStorage.getItem("email")).subscribe();

Before doing that I want to check the db if the email exsists?

So i have another service that will check the API for the id of that email in the database:

this.housingService.getNewsletterId(email).subscribe();

How to check if the result of the above code is returning null or not?