How do I insert unique user ID string in Google Analytics JS code?

Question

I am trying to insert a unique user ID in the google analytics tracking code:

ga(‘set’, ‘&uid’, {{USER_ID}}); // Set the user ID using signed-in user_id.

I have looked in the browser run time environment and I see some QA variables.. qa_root, qa_request, etc. 

but I don't see a variable for user ID..    

what's the easiest way to get a string variable that I can put in the GA  code? 

Thanks !