当我尝试对控制号进行编码时，为什么会收到无法读取的属性消息

Question

我是编码和编程的新手，我一直在Youtube上关注一段程序员使用HTML、CSS和Javascript制作响应缩略图滑块的屏幕记录
我已经到了一个地步，我正试图获得要在页面上显示的幻灯片的控制编号，但我仍然收到以下错误消息：

script.js:48 Uncaught TypeError：无法在script.js:51 [e-mail]:48 [e-mail]:23(匿名(@script.js:51"读取开始时(script.js:44(为null的属性'appendChild'

有人能告诉我为什么会发生这种情况吗？我的代码在下面。

const controls = document.querySelector(".controls");
const container = document.querySelector(".thumbnailcontainer");
const allBox = container.children;
const containerWidth = container.offsetWidth;
const margin = 30;
var item = 0;
var totalItems = 0;
// item setup per slide
responsive = 
[ { breakPoint: { width:    0, item: 1 }} // if width greater than    0 (1 will item show)
, { breakPoint: { width:  600, item: 2 }} // if width greater than  600 (2 will item show)
, { breakPoint: { width: 1000, item: 6 }} // if width greater than 1000 (4 will item show)
] 
function load() {
for (let i = 0; i < responsive.length; i++) {
if (window.innerWidth > responsive[i].breakPoint.width) {
items = responsive[i].breakPoint.item
}
}
start();
}
function start() {
var totalItemsWidth = 0
for (let i = 0; i < allBox.length; i++) {
//width and margin setup of items
allBox[i].style.width = (containerWidth / items) - margin + "px";
allBox[i].style.margin = (margin / 2) + "px";
totalItemsWidth += containerWidth / items;
totalItems++;
}
//tumbnail-container width set up
container.style.width = totalItemsWidth + "px";
//slides controls number set up
const allSlides = Math.ceil(totalItems / items);
const ul = document.createElement("ul");
for (let i = 1; i <= allSlides; i++) {
const li = document.createElement("li");
li.id = i;
li.innerHTML = i;
li.setAttribute("onclick", "controlSlides(this)");
ul.appendChild(li);
}
controls.appendChild(ul);
}
window.onload = load();

* {
box-sizing: border-box;
}
.thumbnailslider {
width: 100%;
float: left;
overflow: hidden;
}
.thumbanilslider .thumbnailcontainer {
width: 100%;
float: left;
}
.thumbnailslider .artitem {
height: 175px;
background-color: grey;
line-height: 175px;
text-align: center;
font-size: 50px;
color: #ffffff;
float: left;
}

<div class="thumbnailslider">
<div class="thumbnailcontainer">
<div class="artitem">  1 </div>
<div class="artitem">  2 </div>
<div class="artitem">  3 </div>
<div class="artitem">  4 </div>
<div class="artitem">  5 </div>
<div class="artitem">  6 </div>
<div class="artitem">  7 </div>
<div class="artitem">  8 </div>
<div class="artitem">  9 </div>
<div class="artitem"> 10 </div>
</div>
<!-- Control Slides -->
<div class="artcontrols">
</div>
</div>

Answer 1

您有错误的类名

const controls = document.querySelector(".controls");

const container = document.querySelector(".thumbnailcontainer");
const allBox = container.children;
const containerWidth = container.offsetWidth;
const margin = 30;
var item = 0;
var totalItems = 0;
// item setup per slide
responsive = 
[ { breakPoint: { width:    0, item: 1 }} // if width greater than    0 (1 will item show)
, { breakPoint: { width:  600, item: 2 }} // if width greater than  600 (2 will item show)
, { breakPoint: { width: 1000, item: 6 }} // if width greater than 1000 (4 will item show)
] 
function load() {
for (let i = 0; i < responsive.length; i++) {
if (window.innerWidth > responsive[i].breakPoint.width) {
items = responsive[i].breakPoint.item
}
}
start();
}
function start() {
var totalItemsWidth = 0
for (let i = 0; i < allBox.length; i++) {
//width and margin setup of items
allBox[i].style.width = (containerWidth / items) - margin + "px";
allBox[i].style.margin = (margin / 2) + "px";
totalItemsWidth += containerWidth / items;
totalItems++;
}
//tumbnail-container width set up
container.style.width = totalItemsWidth + "px";
//slides controls number set up
const allSlides = Math.ceil(totalItems / items);
const ul = document.createElement("ul");
for (let i = 1; i <= allSlides; i++) {
const li = document.createElement("li");
li.id = i;
li.innerHTML = i;
li.setAttribute("onclick", "controlSlides(this)");
ul.appendChild(li);
}
controls.appendChild(ul);
}
window.onload = load();

* {
box-sizing: border-box;
}
.thumbnailslider {
width: 100%;
float: left;
overflow: hidden;
}
.thumbanilslider .thumbnailcontainer {
width: 100%;
float: left;
}
.thumbnailslider .artitem {
height: 175px;
background-color: grey;
line-height: 175px;
text-align: center;
font-size: 50px;
color: #ffffff;
float: left;
}

<div class="thumbnailslider">
<div class="thumbnailcontainer">
<div class="artitem">  1 </div>
<div class="artitem">  2 </div>
<div class="artitem">  3 </div>
<div class="artitem">  4 </div>
<div class="artitem">  5 </div>
<div class="artitem">  6 </div>
<div class="artitem">  7 </div>
<div class="artitem">  8 </div>
<div class="artitem">  9 </div>
<div class="artitem"> 10 </div>
</div>
<!-- Control Slides -->
<div class="controls">
</div>
</div>