Hi Sourabh
there are multiple objects that you can use in client script.
1. g_user - to get user information
2. g_form - to get field information
3. g_list - to get list objects
4. g_menu is the UI Context Menu that is about to be shown. The onShow script can make changes to the appearance of the menu before it is displayed using these methods
5.g_item is the current UI Context Menu item that is about to be shown. It is used in several of the g_menu methods to specify an item.
Check below link for more details
Client Scripts - ServiceNow Wiki
