Adding and removing properties to an object leaks memory. A test program (adapted from node's test/simple/test-event-emitter-memory-leak.js) follows -

var Thread = java.lang.Thread;
var runtime = java.lang.Runtime.getRuntime();
var mb = 1024 * 1024;

var noop = function() {};
var map = {};

runtime.gc();
Thread.sleep(1000);
var before = runtime.freeMemory();
print('before ' + before / mb);
for (var i = 0; i < 16e5; ++i) {
  var name = 'a-pretty-long-event-name-zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz-' + i;
  map[name] = noop;
  delete map[name];
  if ((i % 1e5) === 0) {
    runtime.gc();
    print(' ' + (runtime.freeMemory() - before) / mb);
  }
}
runtime.gc();
Thread.sleep(1000);
var after = runtime.freeMemory();
print('after ' + after / mb);
print('leaked ' + ((after - before) / mb));

when run as 'jjs -J-Xms1g -J-Xmx1g leak.js' can be observed to increase its resident size steadily as the program progresses, and prints freeMemory which is seen as steadily decreasing -

before 966.2640838623047
       7.783241271972656
       44.487510681152344
       45.94429016113281
       46.042991638183594
       45.18035125732422
       44.3046875
       43.93107604980469
       43.05583953857422
       42.19343566894531
       41.37696075439453
       40.98670196533203
       40.12110137939453
       39.256675720214844
       38.39038848876953
       37.998321533203125
       37.12486267089844
after 1001.1347503662109
leaked 34.87066650390625