Javascript 函数未显示

Question

我正在为学校做一个元素，我的 Javascript 突然停止显示。谁能告诉我发生了什么事？

我说的是应该出现在右下角的滚动回到顶部按钮。

jsfiddle:https://jsfiddle.net/u67cea52/

$(document).ready(function() {
function showItem(evt, itemName) {
var i, tabcontent, tablinks;
tabcontent = document.getElementsByClassName("tabcontent");
for (i = 0; i < tabcontent.length; i++) {
    tabcontent[i].style.display = "none";
}
tablinks = document.getElementsByClassName("tablinks");
for (i = 0; i < tablinks.length; i++) {
    tablinks[i].className = tablinks[i].className.replace(" active",     "");
}
document.getElementById(itemName).style.display = "block";
evt.currentTarget.className += " active";
}
$(document).ready(function(){

//kijkt of je verder bent dan de cover pagina, zoja dan verschijnt de knop
$(window).scroll(function(){
    if ($(this).scrollTop() > 100) {
        $('.scrollToTop').fadeIn();
    } else {
        $('.scrollToTop').fadeOut();
    }
});

//het event wat ervoor zorgt dat als je op de knop klikt dat je weer     naar boven gaat
    $('.scrollToTop').click(function(){
        $('html, body').animate({scrollTop : 0},800);
        return false;
});

});
});

Answer 1

这可能会有所帮助:

HTML:

<div id="content">Hello World</div>
<a href="#" id="back-to-top" title="Back to top">&uarr;</a>

CSS:

#back-to-top {
    position: fixed;
    bottom: 40px;
    right: 40px;
    z-index: 9999;
    width: 32px;
    height: 32px;
    text-align: center;
    line-height: 30px;
    background: #f5f5f5;
    color: #444;
    cursor: pointer;
    border: 0;
    border-radius: 2px;
    text-decoration: none;
    transition: opacity 0.2s ease-out;
    opacity: 0;
}
#back-to-top:hover {
    background: #e9ebec;
}
#back-to-top.show {
    opacity: 1;
}
#content {
    height: 2000px;
}

JS:

if ($('#back-to-top').length) {
    var scrollTrigger = 100, // px
        backToTop = function () {
            var scrollTop = $(window).scrollTop();
            if (scrollTop > scrollTrigger) {
                $('#back-to-top').addClass('show');
            } else {
                $('#back-to-top').removeClass('show');
            }
        };
    backToTop();
    $(window).on('scroll', function () {
        backToTop();
    });
    $('#back-to-top').on('click', function (e) {
        e.preventDefault();
        $('html,body').animate({
            scrollTop: 0
        }, 700);
    });
}