I have created one calendar list for a meeting room in SharePoint 2013. How can I warn the user for double booking and overlapping events without using C# code?
Sharepoint 2013 has a very rich java script object model. This gets even easier to use with third party libraries such as spservices. Worth looking into this.
You could use the already existing java script function PreSaveAction() which allows you to inject your own validation to a form. Using this you could query the current calendar and see if there are any conflicts with the date range the user has selected. If there is a conflict the PreSaveAction function can be set to return false so the item is not saved and an error message displayed. This will allow the user to then change the date range before saving again.
If you need any more guidance on this I'd he happy to help
In your PreSaveAction() function you can query your calendar and find appointments that conflict with the current appointment being made. If there is a conflict you would tell your function return false and also display some error message. If there is no conflict it will return true and SharePoint can save the list item.
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Hope this helps