We recently implemented alias at signup and then call identify upon login so that we can track a single user pre and post signup auth. It appears that we're now doing that correctly as the distinct_id for a user pre and post auth are now the same value. However, since we've put this into place, the Mixpanel default $user_id property that gets tracked with default events like $ae_session is now coming back as 'undefined' for newly registered users. Is there an additional step that needs to happen in order for $user_id to be populated with the user's distinct_id or our internal ID?
Best answer by cherise
You do not need to do anything additional here! The $user_id value is the unique ID that mixpanel.identify() sets, so until the newly registered users come back to login and trigger mixpanel.identify(), it will remain undefined.
Here is a link to all default properties that we track with our client-side SDKs.
I hope this helps!