Snapshot
Represents a read-only ShareDB document at a particular version number.
Snapshots can not be used to manipulate the current version of the document stored in the database. That should be achieved by using a Doc.
Properties
  type – string 
 The URI of the document type
Document types can change between versions if the document is deleted, and created again.
  data – Object 
 The snapshot data
  v – number 
 The snapshot version