You are right, JScript used by Opus scripting is outdated version of JavaScript, but it is better to learn JScript than the VBScript since its structure is more modern so getting used to it is more broadly useful.
For more information about the Microsoft's JScript version used in Opus see this awesome post: Here are the Windows Script Host / ActiveScript Reference Manuals
Quick tip: since the Opus is using the Microsoft's WSH (Windows Script Host) for scripting support, you can search JScript WSH to find other interesting/advanced examples on the web that work in Opus too.