These docs are for an old release. Info on upgrading to v6
edit doc

eventDataTransform

A hook for transforming custom data into a standard Event object.

function( eventData ) {}

This hook allows you to receive arbitrary event data from a JSON feed or any other Event Source and transform it into the type of data FullCalendar accepts. This let’s you easily accept alternative data formats without having to write a completely custom events function.

This function is called once per received event. eventData is the event data that has been receieved. The function must return a parsable Event object or false, which will discard the event.

This function is not called for the addEvent method.

If you want to receive a non-standard root object from your event sources that is not an array, return a value from your eventSourceSuccess callback.