Question

Anyways using sync method only in collection or only in model can suffice, then why do they have to sync at both the places?

Answer 1

Actually both sync() methods are just proxies to a common Backbone.sync() method:

• Model.sync()
• Collection.sync()
• Backbone.sync()

Collection always delegate in the Model.sync() for individual operations over its individual models like: create, remove and so on. But Collection uses its own sync() in the fetch() operation due it is very different to fetch a Model or a Collection, for example: the URL follows another patter and the backend layer should respond different.

In the other hand I see the Backbone.sync() as a private method and I try to not use it directly, if I’m doing this I don’t feel well. I think the sync() method is a handler point to allow you to overwrite completely the backend synchronization a method that you can overwrite to implement different persistance layers as for example using LocalStorage. But not for be called directly.

As @JMM has said in the comments, the Model.sync() and Collection.sync() is also a good point to be overwrote to make it “does something custom and then calls Backbone.sync() to carry on as usual”.