Sharepoint search within Excel Add-In

We have an excel add in developed in the old days of VB Script, this uses some custom web services that are deployed into sharepoint server 2013 which relies on search to get the results

I know excel add ins are done with javascript and therefore I could just use the REST API to talk with Sharepoint.

However this add-ins run on the PC, so I am not sure about which user context will the rest queries run on, and if I need some kind of authentication coding?