Iterate over checkboxes array to determine if checked

I have a table with a "select all" checkbox. When I populate this table, all of the items that have been selected previously are checked. Currently, the "select all" checkbox will return checked even if not all of items are checked. I'm trying to create a function that will iterate over each checkbox, and if all are checked, then it will automatically toggle the select all checkbox. Same logic if not all items are selected - the select all checkbox will not be selected.

Here's the code I have so far:

function unclickSelectAll(parent) {
var $checkboxes = parent.find('input[type="checkbox"]');
var $selectAllCheckbox =  $checkboxes.filter('.vendor');
var $invoiceCheckBoxes = $checkboxes.filter('.invoice');
$invoiceCheckBoxes.each(function(i, c) {
    var checkbox = $(this);
    if (checkbox[i].checked) {
        $selectAllCheckbox.prop('checked', true);
    } else { 
    $selectAllCheckbox.prop('checked', false);
 }
});

};

What happens currently is that it will loop over the first checkbox, and if it's checked it will change the checked property of the select all check box to true. Then through the next loop I get an error that it cannot find the 'checked' property of checkbox[i];