How to set pageItem properties in Basil.js?

Question

This is a follow up on this question.

How do I set pageItem properties like shearAngle in Basil.js? In ExtendScript it is:

myPageItem.properties = {
    shearAngle:20
};

or

myPageItem.shearAngle = 20;

What would be the Basil.js-ish equivalent?

What about nested properties like some preferences?

app.activeDocument.properties = {
    documentPreferences:{
        pageWidth:100,
        pageHeight:100
    }
};