the same as title...
Since ExtJS is built on top of Ext Core which is hugely tied to DOM, there's little sense in trying to use ExtJS server-side. However, there are a lot of low level stuff in ExtJS that could be used to make a very nice server-side API/Library. For example, the whole OO Object scheme and Ext.extend() and Ext.isArray() type functions. The data stores and records are also not so tied to the DOM that they would be a good addition to a server-side API.
Ext.Mdoel can be used in SSJS, too?
I'm not sure if ExtJS can be loaded server-side. It would have to comply with the CommonJS module scheme or the server-side environment would have to support the DOM (like Jaxer).
However, something like Ext.Model is pretty smart to do server-side. Perhaps Ext.Model can be extracted from the ExtJS sources in a way that makes it work server-side.