Sys.WebForms.PageRequestManager isInAsyncPostBack Property
Returns a value that indicates whether the PageRequestManager object is processing a postback.
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 status = Sys.WebForms.PageRequestManager.getInstance().get_isInAsyncPostBack();
You can use the isInAsyncPostBack property to define how the PageRequestManager class behaves when a user submits a postback multiple times on a page.
The following example shows how to determine whether the PageRequestManager is currently processing a postback.