JavaScript Editor is not working OOTB in Eclipse/devstudio installed via RPM. It fails because of missing JSDT/Equinox hook for Nashorn in eclipse.ini. See https://wiki.eclipse.org/JSDT/Equinox_hook_for_Nashorn
The same error occurs if JSDT installed to an existing Eclipse via p2. So it's rather an upstream issue.

The following line added to eclipse.ini fixes the problem:

-Dosgi.framework.extensions=org.eclipse.wst.jsdt.nashorn.extension

It works OOTB in devstudio installed via our installer because we have this hook in devstudio.ini