Подсчет количества чекбоксов на jQuery

PHP
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var count = 0;
$(function() {
     updateCount();
     $('input[type=checkbox]').change(function() {
           updateCount(this.checked ? 1 : -1);
     });
     $('#invert').click(function(e) {
           e = $('input[type=checkbox]');
           e.each(function(i, el) {
                 el.checked = !el.checked;
           });
           updateCount(e.length - count - count);
     });
});
function updateCount(a) {
     count = a ? count + a : $('input[type=checkbox]:checked').length;
     $('#count').text(count);
}

https://ru.stackoverflow.com/questions/29719/%D0%9F%D0%BE%D0%B4%D1%81%D1%87%D0%B5%D1%82-%D0%BA%D0%BE%D0%BB%D0%B8%D1%87%D0%B5%D1%81%D1%82%D0%B2%D0%B0-%D1%87%D0%B5%D0%BA%D0%B1%D0%BE%D0%BA%D1%81%D0%BE%D0%B2-%D0%BD%D0%B0-jquery