jQuery - Append a Value to a INPUT, keeping it a Comma Delimited list

I have an input like follows:

<input type="hidden" id="attachment-uuids" value="">

I'd like to be able to append a value to the Input, at different times:

$('#attachment-uuids).val('55555');

results in:

<input type="hidden" id="attachment-uuids" value="55555">

But then doing:

$('#attachment-uuids).val('66666');

results in:

<input type="hidden" id="attachment-uuids" value="66666">

Where I'd like the following:

<input type="hidden" id="attachment-uuids" value="55555, 66666">

how can I append values when the value is empty and when the value is not empty with a comma delimited list?

Thanks