jQuery按类计数元素-实现此目的的最佳方法是什么？

应该只是这样：

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

附带说明一下，在链接对jQuery对象的许多函数之前，先检查length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');    });}