var f = xdmp.function(xs.QName("fn:empty"));
xdmp.apply(f, []);

  => true

var f = xdmp.function(xs.QName("fn:concat"));
xdmp.apply(f, "hello", " world");

=> hello world

Given square.sjs in the modules database as the following:

module.exports = function(width) {
  return {
    area: function() {
      return width * width;
    },
    set: function(_width) {
      width = _width;
    }
  };
}

var mySquare = xdmp.apply(xdmp.function(null,"square.sjs"),4);
xdmp.apply(mySquare.set, 3);
xdmp.apply(mySquare.area)

=> 9