Gets a value that indicates whether the application is in the process of disposing its resources. This member is static and can be invoked without creating an instance of the class.
To get or set property values for client API properties, you must call property accessor methods that are named with the get_ and set_ prefixes. For example, to get or set a value for a property such as cancel, you call the get_cancel or set_cancel methods.
var isDisposing = Sys.Application.get_isDisposing();
true if the application is disposing its resources; otherwise, false.