jQuery fullCalendar displayed undefined on title

I was having the same issue after upgrading fullCalendar.js from v2.6.1 to v3.4.0

In my case the Issue resolved by including fullcalendar.js & scheduler.min.js after moment.js

I was having the same issue after upgrading fullCalendar, took me a bit to figure out because for almost a year everything has been working fine and I had upgraded fullCalendar in the past without any issues, for some reason I had to include moment.js in the page I was using the fullCalendar on, see I run an MVC site and previously the master page (_layout.cshtml) was referencing moment.js, not sure right now why that doesn't work anymore, just as a test I added a reference to moment in the actual page I use fullCalendar and the undefindundefined went away and so did another issue I was having with events.

In my case the fix was:

@Scripts.Render("~/bundles/dates") 

in your case it may just be:

<script src='//cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js'></script>

I have the same issue with fullcalendar v3.4.0 and fullcalendar-scheduler v1.6.2 within an Angular2 component. I downgraded to last functioning version fullcalendar v3.1.0. This issue seems to be introduced above fullcalendar v3.2.0

I was having the same issue, but for me it was not about moment.js.

I use node_modules and the loading sequence was like that :

require('fullcalendar');
require('fullcalendar-scheduler');

But, after more investigation, I found that fullcalendar-scheduler was already loading fullcalendar module, so I just had to keep the scheduler and all is working fine :

require('fullcalendar-scheduler');