In this doc: https://github.com/snowplow/snowplow/wiki/snowplow-tracker-protocol#event2
The event ID (eid) is the unique identifier (UUID) for this row. Historically we generated this in the Enrichment process but where possible we are migrating this to being generated in the trackers.
It appears that the event id is in fact being generated by the js tracker but I'm not sure if this is always the case?
We are looking for a way to extract the event id that is generated and passed with the pageview event and pass it with a 3rd party event that we fire later in the same page load. Is this event id saved in the js object or cookies in any way that it would still be available after the pageview event is completed?