JavaScript -- for里面的定时器

let length = $('.swiper-nav li').length;
for (let i = 0; i < length; i++) {
     let item = $('.swiper-nav li').eq(i);
     (function (i) {
           setTimeout(function () {
                  item.addClass('tracking-in-contract-bck')
           }, 1000 * i);
     })(i)
}

这样就是闭包，快乐~