There's at least one way

var els = document.querySelectorAll('#divConfirm table')[1].querySelectorAll('tr');



var last = [].slice.call(els).pop();

but, the following statement

But if I do not know the length prior to running the script

makes no sense, you already have the collection of elements, so you would always know the length

var els = document.querySelectorAll('#divConfirm table')[1].querySelectorAll('tr');



var last = els[els.length - 1];

Another option as the8472's answer suggests would be

document.querySelector('#divConfirm table:nth-child(2) tr:last-child');