Working with Cookies, Establish API connection in Javascript (Senior Level)

Hello,

I am looking for a high experienced Javascript Developer who can use 3rd party API and make connections by checking cookies.

This application will be a small standalone script which will be implemented in a few pages.

This script needs to check if there is an active cookie with a signed in user.
If yes, a popup should appear. If not, it shouldn't appear.

Please only qualified people.

Thanks