I solved this. The documentation is incorrect for ASP.NET Web Forms.
It tells you to use the following code:
EventClickHandling="JavaScript"
EventClickJavaScript="editEvent(e.id);"
But this doesn't work. It needs to read as:
EventClickHandling="JavaScript"
EventClickJavaScript="editEvent(e.data.id);"
It is missing the "data" in the example.