我有三个下拉菜单项，当我第一次单击每个菜单项时，按预期的方式工作，但是当我第二次单击其中的一个菜单项时，它会滚动到顶部，然后切换到适当的选项卡-面板。为什么它第一次能按预期工作，但第二次却不正确？我的HTML位于代码段的下方，但我无法让jQuery不会在代码段中引起错误，因此该代码段分别位于下方。
<!DOCTYPE html>
<html lang="en" dir="ltr">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<!-- <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script> -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
<!-- import jQuery scrolling file below as scrolling.js -->
<script src="js/scrolling.js"></script>
</head>
<body>
<!-- ********** MAIN CONTENT ********** -->
<div id="main">
<section class="background-image d-flex align-items-center" id="home">
<div class="container-fluid">
<div class="row">
<div class="col-xl-1 col-lg-1 col-md-1 col-sm-0 col-xs-0"></div>
<div class="col-xl-6 col-lg-6 col-md-6 col-sm-12 col-xs-12">
<h1 class="home-header">Hello!</h1>
<br><br><br><br><br><br><br><br><br><br>
</div>
<div class="col-xl-5 col-lg-5 col-md-5 col-sm-0 col-xs-0"></div>
</div>
</div>
</section>
<!-- ********** ABOUT ME ********** -->
<section class="container-fluid" id="about">
<div class="row">
<div class="col-xl-1 col-lg-1 col-md-1 col-sm-0 col-xs-0"></div>
<div class="col-xl-10 col-lg-10 col-md-10 col-sm-12 col-xs-12">
<h1>About Me</h1>
<br><br><br><br><br><br><br><br><br><br>
<div class="col-xl-1 col-lg-1 col-md-1 col-sm-0 col-xs-0"></div>
</div>
</div>
</section>
<!-- ********** EXPERIENCE ********** -->
<section class="container-fluid" id="experience">
<div class="row">
<div class="col-xl-1 col-lg-1 col-md-1 col-sm-0 col-xs-0"></div>
<div class="col-xl-10 col-lg-10 col-md-10 col-sm-12 col-xs-12">
<h1>Experience</h1>
<br><br><br><br><br><br><br><br><br><br>
</div>
<div class="col-xl-1 col-lg-1 col-md-1 col-sm-0 col-xs-0"></div>
</div>
</section>
<!-- ********** PROJECTS ********** -->
<section id="projects">
<div class="container-fluid">
<div class="row">
<div class="col-xl-1 col-lg-1 col-md-1 col-sm-0 col-xs-0"></div>
<div class="col-xl-10 col-lg-10 col-md-10 col-sm-12 col-xs-12">
<h1>Projects</h1>
</div>
<div class="col-xl-1 col-lg-1 col-md-1 col-sm-0 col-xs-0"></div>
</div>
</div>
<div>
<ul class="nav nav-tabs nav-justified">
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-haspopup="true" aria-expanded="false" id="dropdown">Projects</a>
<div class="dropdown-menu dropdown-menu-center">
<a class="dropdown-item" data-toggle="tab" href="#tab1">tab1</a>
<a class="dropdown-item" data-toggle="tab" href="#tab2">tab2</a>
<a class="dropdown-item" data-toggle="tab" href="#tab3">tab3</a>
</div>
</li>
</ul>
</div>
<div class="tab-content">
<div class="tab-pane container fade show active" role="tabpanel" id="tab1">
<div class="row">
<div class="col-xl-8 col-lg-8 col-md-8 col-sm-12 col-xs-12">
<h3>tab1</h3>
</div>
</div>
</div>
<div role="tabpanel" class="tab-pane container fade" id="tab2">
<div class="row">
<div class="col-xl-8 col-lg-8 col-md-8 col-sm-12 col-xs-12">
<h3>tab2</h3>
</div>
</div>
</div>
<div role="tabpanel" class="tab-pane container fade" id="tab3">
<div class="row">
<div class="col-xl-8 col-lg-8 col-md-8 col-sm-12 col-xs-12">
<h3>tab3</h3>
</div>
</div>
</div>
</div>
</section>
</div>
</body>
</html>
我的jQuery：
$(document).ready(function() {
$("a").on('click', function(e) {
let ref = jQuery(this).attr("class");
if (this.getAttribute("href").charAt(0) === "#" && (ref !== "nav-link" && ref !== "dropdown-item")) {
e.preventDefault();
var hash = this.hash;
$('html, body').animate({
scrollTop: $(hash).offset().top
}, 800, function() {
window.location.hash = hash;
});
}
});
});