打开页面，数字会自增的效果怎么弄？

<div class="container position-absolute l-numberfontsizebox">
    <div class="row pt-5 l-help">
        <div class="col-4  text-center">
            <p class="text-gray l-numberfontsize">总考试次数</p>
            <p class="text-light l-numberfontsize2">100+</p>
        </div>
        <div class="col-4  text-center">
            <p class="text-gray l-numberfontsize">总考试次数</p>
            <p class="text-light l-numberfontsize2">50+</p>
        </div>
        <div class="col-4 text-light text-center">
            <p class="text-gray l-numberfontsize">总考试次数</p>
            <p class="text-light l-numberfontsize2">59934686+</p>
        </div>
    </div>
</div>

<script>
    $(function(){
        $(".l-numberfontsize2").each(function(){
            var _this = this;
            var num = parseInt($(this).text());
            var a = 0;
            setInterval(function(){
                if(a < num )
                {
                    a += parseInt(num/50);
                    if(a > num)
                    {
                        $(_this).html(num);
                    }else {
                        $(_this).html(a);
                    }
                }
            } , 50);
        })
    })

</script>