function readOnly(count){ }
Starting November 20, the site will be set to read-only. On December 4, 2023,
forum discussions will move to the Trailblazer Community.
+ Start a Discussion
Prabitha SPrabitha S 

please tell Syntax for upsert & undelete trigger & Purpose undelete?

SonamSonam (Salesforce Developers) 
Following thread has a code snippet for the undelete trigger with the use case defined as well which will help you understand the purpose of undelete:
https://developer.salesforce.com/forums/ForumsMain?id=906F00000008wWXIAY