JavaScript Cannot read property 'classList' of undefined Error

Question

I was working on a personal project of making a slider using pure javascript. But The js code is not working. When I click the next button, the console shows an error that Uncaught TypeError: Cannot read property 'classList' of undefined. Same with the Previous button. I tried everything I could but I am not able to find the answer. =

JS Code:

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 Code:

<!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();
});

If you get more undefined errors, find the lines where it happens and add the if or && checks like I did before trying to access a property on it

JavaScript Cannot read property 'classList' of undefined Error

Question

2 answers

solution1
0 2018-06-04 04:54:46

solution2
0 2018-06-04 05:21:00

JavaScript Cannot read property 'classList' of undefined Error

Question

2 answers

solution1 0 2018-06-04 04:54:46

solution2 0 2018-06-04 05:21:00

solution1
0 2018-06-04 04:54:46

solution2
0 2018-06-04 05:21:00