I normally turn to ringojs[1] for "off-browser javascript" with Java integration. It's been around nearly 20 years[2] now! Is this a new take on the same concept? The examples seem web application focused, but there's not even a mention of JSGI[3] or whether it was even considered.
Not to mention there are dozens of other frameworks that support using Javascript directly like Vert.x, Jooby, Wicket (vaguely remember), Spring and even raw servlets and netty can all be programmed in Javascript using Rhino or Nashhorn.