It's a piece of artwork with simplicity, I love this project.
In some cases, it's better to give the option to user of rounding options:
like:
t.is(format("#,##0.00", "123456.789"), "123,456.79");
t.is(format("#,##0.00", "123456.789", { rounding: 'floor'} ), "123,456.78");