##為當前 li下的a 新增樣式
html:

 <ul id="head_nav_ul">
            <li class="head_nav_li">
                <a href="/products" class="product_li_a">產品</a>
            </li>
            <li>
                <a href="/price" class="product_li_a">價格</a>
            </li>
            <li>
                <a href="/cases" class="product_li_a">案例</a>
            </li>
</ul>
 

js：

$(document).ready(function () {
    $("#header_ul li a").each(function () {
         $this = $(this);
       if ($this[0].href == String(window.location)) {
             $this.addClass("active");
       }
  });
});