Retrieve fields from related entity using Jscript

Good afternoon,

I'm looking for a way to retrieve fields from a related entity using Jscript.

I've had a look around the internet and it appears a lot of their solutions are prior to Dynamics 365 using OData feeds and unsupported plugins.

Does anyone have a script that works in Dynamics 365 Online?

I would prefer to steer clear of workflows as I want to retrieve the information instantly, rather than on save.

Thanks in advance!