JavaScript无法读取属性'；类列表'；未定义错误的定义

我在做一个个人项目，用纯javascript制作一个滑块。但是js代码不起作用。单击“下一步”按钮时，控制台显示一个错误，该错误为未捕获的TypeError:无法读取未定义的属性“classList”。与上一个按钮相同。我尽了我所能，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();
});

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.queryselectoral（“li”）；
var active=document.getElementsByClassName（“active”）[0]；
对于（var i=0；i当您定义
active
元素var时，没有元素具有
active
类-将
active
作为类添加到html中的第一个项中。
var next1=document.getElementById（“下一步”）；
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();
});

var prev1=document.getElementById（“prev”）；
var ulist=document.getElementsByTagName（“ul”）.lastChild；
var ulist1=document.getElementsByTagName（“ul”）.firstChild；
var slides=document.queryselectoral（“li”）；
对于（var i=0；iOn），该错误出现在哪一行？您能发布更完整的代码以便我们有一个可以使用的代码吗？您能与我们共享您的标记吗？main.js:36 Uncaught TypeError:无法在HtmleElement的prev（main.js:36）读取null属性“classList”。（main.js:46）在JS文件中，必须将变量设置为全局变量，以便在函数中读取。请尝试删除“var”。
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();
});