title: Set Google Analytics User ID after creating the tracker
body: <p>The Google Analytics User ID <a href="https://developers.google.com/analytics/devguides/collection/analyticsjs/user-id" rel="noreferrer">documentation page</a> shows that you set the User ID when you create the tracker. I have a javascript only project with sign-in after page load, is there any way to set the User ID after login? Something like <code>ga("set", "userid", "UNIQUEID")</code></p>
  
  <p>On the <a href="https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference#create" rel="noreferrer">field reference page</a> it says you can only set user-id in the create function. Are we able to call create again with the user-id? Will that create a new tracker, or override the old one?</p>
  
  <p><strong>EDIT:</strong> The beta version of User ID tracking showed that you could specify the userid after creating the tracker using <code>ga('set', '&amp;uid', {{ USER_ID }});</code> and that would try to unify the session from before the userid was set with the one after. That seems to be very different than what the docs current specify. Is there any way to do this with the current method?</p>