I have a form with additional form action buttons along with the default save and cancel. One is a reject button and the other is "bypass". I have the aforementioned "bypass" to "commit"and "redirect" but I would also like it to update a field not on the form ( but same list ) to a particular choice selection value. Is this possible?
To update via JSCOM, you can modify the function found here: https://msdn.microsoft.com/en-us/library/office/hh185011(v=office.14).aspx#sectionSection2
A good read for updating via REST (a good practice to start getting into) can be found here: http://www.codeproject.com/Articles/990131/CRUD-Operation-to-List-Using-SharePoint-Rest-API
Hope this helps.