currentSelection.js

var System = java.lang.System;
var SelectionChangedListener = org.openstreetmap.josm.data.SelectionChangedListener;
var DataSet = org.openstreetmap.josm.data.osm.DataSet;

var SelectionAdapter = {
  init: function() {
    DataSet.addSelectionListener(this);
  },

  selectionChanged: function (newSelection) {
    System.out.println(newSelection);
  }
}
var s = new SelectionChangedListener(SelectionAdapter);
s.init();