JS window.addEventListener仅适用于一页

Question

我有一个javascript，如果元素在屏幕上，则由window.addEventListener触发。当查看特定的div时，会将CSS类（来自Animate CSS）添加到div元素。该代码仅在我网站的一页上有效，而在其他网页上则无效。

我仅在第二页上发现此控制台错误（JS不起作用）

TypeError: $elem.offset(...) is undefined

Live Project URL：

// Element In View function

function isOnScreen(elem) 
    // if the element doesn't exist, abort
    if (elem.length == 0) 
        return;
    
    var $window = jQuery(window)
    var viewport_top = $window.scrollTop()
    var viewport_height = $window.height()
    var viewport_bottom = viewport_top + viewport_height
    var $elem = jQuery(elem)
    var top = $elem.offset().top
    var height = $elem.height()
    var bottom = top + height

    return (bottom > viewport_top) && (top < viewport_bottom)


// Animation when element is in view

window.addEventListener('scroll', function (e) 
    if (isOnScreen('#lfdesign')) 
        $("#lfdesign .flexcontainer").addClass("animated slower bounceInRight");
    
    if (isOnScreen('#books')) 
        $("#books .flexcontainer").addClass("animated slower bounceInLeft");
    
    if (isOnScreen('.srp-color')) 
        $(".srp-color .flexcontainer").addClass("animated slower bounceInRight");
    
    if (isOnScreen('.nescol-color')) 
        $(".nescol-color .flexcontainer").addClass("animated slower bounceInLeft");
    
    if (isOnScreen('.worldskills-color')) 
        $(".worldskills-color .flexcontainer").addClass("animated slower bounceInRight");
    
    if (isOnScreen('.silvernote-color')) 
        $(".silvernote-color .flexcontainer").addClass("animated slower bounceInLeft");
    
);

CSS

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/animate.css/3.7.2/animate.min.css">

html

<section class="left silvernote-color">

    <div class="flexcontainer">

        <div class="flex-item">

            <a href="webdesign/silvernote">

                <img class="normal" src="img/silvernote-logo.png" alt="The logo for SilverNote, a music enterprise hub in Aberdeen who I helped to create a responsive site for by adding artist profiles with a team of developers">

            </a>

        </div>

        <div class="flex-item">

            <p>SilverNote Music is a music enterprise hub in Aberdeen that want to promote new music. They tasked me and three others to work as a team to create a responsive site that catered to those viewing on computers as well as a younger, mobile audience.</p>

            <p>My main responsibilities were communicating with our client and developing the artist page. I pitched the prototype to the wider organisation and gathered feedback from the band members. This feedback informed the music players I created on the artist profiles.</p>

        </div>

    </div>

</section>