JavaScript无法读取未定义错误的属性“ classList”

Question

我当时正在从事一个使用纯JavaScript制作滑块的个人项目。 但是js代码无法正常工作。 当我单击下一步按钮时，控制台将显示一个错误，提示Uncaught TypeError：无法读取未定义的属性'classList'。 与上一个按钮相同。 我已尽力而为，但找不到答案。 =

JS代码：

var next1 = document.getElementById("next");
var prev1 = document.getElementById("prev");
var ulist = document.getElementsByTagName("ul").lastChild;
var ulist1 = document.getElementsByTagName("ul").firstChild;
var slides = document.querySelectorAll("li");
var active = document.getElementsByClassName("active")[0];

for(var i=0; i<3; i++){
slides[i].style.display = 'none';
}

slides[0].classList.add("active");

document.getElementsByClassName("active")[0].style.display = "block";

function next() {
    active.classList.add("oldActive");
    active.classList.remove("active");
    if(Object.is(document.getElementsByClassName("oldActive")[0]) , ulist){
        slides[0].classList.add("active");
    }

    else{
        document.getElementsByClassName("oldActive")[0].nextElementSibling.classList.add("active");
    }
}

function prev() {
    document.getElementsByClassName("active")[0].classList.add("oldActive");
    document.getElementsByClassName("active")[0].classList.remove("active");
    if(Object.is(document.getElementsByClassName("oldActive")[0]) , ulist1){
        slides[2].classList.add("active");
    }
    else{
        document.getElementsByClassName("oldActive")[0].previousElementSibling.classList.add("active");
    }
}

next1.addEventListener('click' , function() {
    next();
});


prev1.addEventListener('click' , function() {
    prev();
});

HTML代码：

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Image/Content Slider with Pure JS</title>
    <link rel="stylesheet" href="css/style.css">
    <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.0.13/css/all.css" integrity="sha384-DNOHZ68U8hZfKXOrtjWvjxusGo9WQnrNx2sqG0tfsghAvtVlRW3tvkXWZh58N9jp" crossorigin="anonymous">
</head>
<body>
    <header>
        <h1>A Responsive Image/Content Slider</h1>
        <p>It is made with HTML5, CSS3 and JavaScript</p>
    </header>
    <div class="left">
    <h1><i class="fas fa-arrow-left" id="prev"></i></h1>
    </div>
    <main>

        <div class="slider">
            <ul class="slides" id="slidelist">
                <li class="slide 1" name="slideo"><img src="images/hero_bg.jpeg" alt=""></li>
                <li class="slide 2" name="slideo"><img src="images/hero_bg1.jpeg" alt=""></li>
                <li class="slide 3" name="slideo"><img src="images/hero_bg1.jpeg" alt=""></li>
            </ul>               
        </div>

    </main>
    <div class="right">
    <h1><i class="fas fa-arrow-right"  id="next"></i></h1>
    </div>
    <script type="text/javascript" src="js/main.js"></script>
</body>
</html>

Answer 1

定义active元素var时，没有元素具有active类-将active作为类添加到html中的第一项。

Answer 2

var next1 = document.getElementById("next");
var prev1 = document.getElementById("prev");
var ulist = document.getElementsByTagName("ul").lastChild;
var ulist1 = document.getElementsByTagName("ul").firstChild;
var slides = document.querySelectorAll("li");


for(var i=0; i<3; i++){
slides[i].style.display = 'none';
}

slides[0].classList.add("active");

document.getElementsByClassName("active")[0].style.display = "block";

function next() {
    var active = document.getElementsByClassName("active")[0];
    if(active) {
       active.classList.add("oldActive");
       active.classList.remove("active");
    }
    if(Object.is(document.getElementsByClassName("oldActive")[0]) , ulist){
        slides[0].classList.add("active");
    }

    else{
        document.getElementsByClassName("oldActive")[0].nextElementSibling.classList.add("active");
    }
}

function prev() {
    document.getElementsByClassName("active")[0].classList.add("oldActive");
    document.getElementsByClassName("active")[0].classList.remove("active");
    if(Object.is(document.getElementsByClassName("oldActive")[0]) , ulist1){
        slides[2].classList.add("active");
    }
    else{
        var siblingEl = document.getElementsByClassName("oldActive")[0].previousElementSibling;
        siblingEl && siblingEl.classList.add("active");
    }
}

next1.addEventListener('click' , function() {
    next();
});


prev1.addEventListener('click' , function() {
    prev();
});

如果遇到更多未定义的错误，请查找发生错误的行，并像我在尝试访问其属性之前所做的那样添加if或&&检查