应该是这样的：

// Gets the number of elements with class yourClass
var numItems = $('.yourclass').length

附带说明一下，在链接 jQuery 对象上的大量函数调用之前检查长度属性通常是有益的，以确保我们确实有一些工作要执行。见下文：

var $items = $('.myclass');
// Ensure we have at least one element in $items before setting up animations
// and other resource intensive tasks.
if($items.length)
{
  $items.animate(/* */)
    // It might also be appropriate to check that we have 2 or more
    // elements returned by the filter-call before animating this subset of 
    // items.
    .filter(':odd')
      .animate(/* */)
      .end()
    .promise()
    .then(function () { 
       $items.addClass('all-done');
    });
}