jquery selector on id returns undefined for id attribute

I have an invoice table. When I change a total amount to be paid, I want to update the "custom" amounts for each record in a table. My issue is that I want to only change the payment amounts on items that are selected to be paid (a checkbox is selected)

Here's my markup

<table id="invoicetable" class="table">
    <tr>
        <th>&nbsp;</th>
        <th nowrap>Invoice</th>
        <th nowrap>Invoice Date</th>
        <th nowrap>Due Date</th>
        <th nowrap>Amount Owed</th>
        <th nowrap>Payment Amount</th>
    </tr>
    <tr id="invoice-20110106-1006-3">
        <td>
            <input checked="checked" id="select-invoice-20110106-1006-3" name="select-invoice-20110106-1006-3" title="choose to not pay anything on this invoice by unchecking this box" type="checkbox" value="true" /><input name="select-invoice-20110106-1006-3" type="hidden" value="false" />
            <input id="invoice-invoice-date-20110106-1006-3" name="invoice-invoice-date-20110106-1006-3" type="hidden" value="1/6/2011 12:00:00 AM" />
            <input id="invoice-date-due-20110106-1006-3" name="invoice-date-due-20110106-1006-3" type="hidden" value="1/5/2011 12:00:00 AM" />
            <input id="invoice-open-amount-20110106-1006-3" name="invoice-open-amount-20110106-1006-3" type="hidden" value="281.680000" />
        </td>
        <td nowrap>
            <a id="invoice-detail-link-20110106-1006-3" title="click to view invoice details" reportUrl="/Report.aspx?R=Y7QXaCujl%2fldGD1Dnc%40%2fI1B%2feDe2R5sHaszF3VEPpfd31lbsCoJ%2fh2KBORgR4dDx1N%2frHXpmd8GJljmMluJJB%4028%40tkZ%40zJLKKHflYpaLRs*" printableUrl="/Report.aspx?R=Y7QXaCujl%2fldGD1Dnc%40%2fIyRhg5x0ZGIY2AmKFLQWmSVJd1VYBWOs7HG7S0%2fw3WyUlG6lL6XS5jQZm7q1Y52DJv%2f0My5R%2fZ5Ecfet%2fREtaoc*">20110106-1006-3</a>
        </td>
        <td nowrap>1/6/2011</td>
        <td nowrap>
            <span title="this invoice is past due" class="pastdue">1/5/2011</span>
        </td>
        <td nowrap>$281.68</td>
        <td nowrap><a title="click to change payment amount" id="pay-invoice-20110106-1006-3">$281.68</a>
            <span id="invoice-payment-20110106-1006-3" style="display:none">
                <div style="float:left"&开发者_C百科gt;<input class="small invoice-payment-amount" id="payment-amount-20110106-1006-3" name="payment-amount-20110106-1006-3" type="text" value="0" /></div> 
                <div style="float:left"><img height="1" src="/Content/Images/Spacer.png" width="6" /><img border="0" id="cancel-payment-20110106-1006-3" src="/Content/images/cross.png" style="cursor:pointer;" title="cancel changes" /></div>
                <input id="full-payment-amount-20110106-1006-3" name="full-payment-amount-20110106-1006-3" type="hidden" value="281.68" />
            </span>
        </td>
    </tr>
</table>

and here is my script

var amountTimer;
$('#Amount').keyup(function () {
    clearTimeout(amountTimer);
    amountTimer = setTimeout(function () {
        var amountString = $('#Amount').val();

        if (IsNumeric(amountString)) {
            var amount = parseFloat(amountString);

            $.each("[id^='select-invoice-']:checked", function () {
                var invoiceId = $(this).attr('id').substr('select-invoice-'.length);
                var openAmountString = $('#invoice-open-amount-' + invoiceId).val();

                if (IsNumeric(openAmountString)) {
                    var openAmount = parseFloat(openAmountString);

                    if (amount >= openAmount) {
                        $('#pay-invoice-' + invoiceId).click();
                        $('#payment-amount-' + invoiceId).val(openAmount.toFixed(2));
                        amount = amount - openAmount;
                    } else if (amount < openAmount && amount > 0) {
                        $('#pay-invoice-' + invoiceId).click();
                        $('#payment-amount-' + invoiceId).val(amount.toFixed(2));
                        amount = 0;
                    } else {
                        $(this).click();
                    }
                }
            });

        }

    }, 1000);
});

There are other events attached to items in here, like fading the entire tr when unchecked, recomputing the total when the textbox changes, and show/hide the textbox on other click events. What happens is the $.each selector works, but $(this).attr('id') returns undefined. This is odd because the selector is working based on the id.

You should be using the other .each():

$("[id^='select-invoice-']:checked").each(function() {
    var invoiceId = $(this).attr('id').substr('select-invoice-'.length);
    /* snip */
});

$.each() is a generic iterator function, for iterating over objects, arrays, and array-like objects.

You're passing a String instead of a jQuery object.

this:

$.each("[id^='select-invoice-']:checked", function () {

would need to be:

$.each($("[id^='select-invoice-']:checked"), function () {