Not sure if this is a KubeJS error, or a Rhino error, but running the following code (or something similar) doesn't work:

let testobject = {
  a: 'bc',
  foo: 'bar'
};
for (const key in testobject) {
  console.log(key);
}

throws a syntax error on line 5, however if the const is replaced with var, it works fine.

Not supported by Rhino https://mozilla.github.io/rhino/compat/engines.html#ES2015-bindings-const-for-in-loop-iteration-scope (see this page for various compat issues)