object with numeric indexes fills up

[Christoph Gritschenberger]

I ran into an oddity when listing the keys of an object today.
If I add a value with a numeric key >=524288 and then add more than one
value with a key <524288, nashorn fills up the space between the lower
indexes.

I got some sample-code here:

    ScriptEngine engine = new
ScriptEngineManager().getEngineByName("javascript");
    Object eval = engine.eval("var x = {}; x[524287] = 'xxx'; x[1] =
'foo'; x[3] = 'bar'; Object.keys(x);");
    System.out.println(((Map) eval).values());
    // prints [1, 3, 524287] as expected
    eval = engine.eval("var x = {}; x[524288] = 'xxx'; x[1] = 'foo';
x[3] = 'bar'; Object.keys(x);");
    System.out.println(((Map) eval).values());
    // prints [1, 2, 3, 524288]

Any idea why that is?

kind regards,
Christoph