问题描述
我一般都不熟悉编程，因此，如果我的问题不是建设性的和明确的，我深表歉意，但我花了一些时间并希望将问题可视化。我测试了Javascript，并希望使用具有基本JavaScript的imageslider。 但是，我无法弄清楚为什么在外部表中编写javascript时我的代码无法起作用。
在页面加载时，imageslider最初不会显示，但是在HTML文件中写入时它可以工作。
这是fiddle
javascript：
var slideIndex = 1;
showSlides(slideIndex);
// Next/prevIoUs controls
function plusSlides(n) {
showSlides(slideIndex += n);
}
// Thumbnail image controls
function currentSlide(n) {
showSlides(slideIndex = n);
}
function showSlides(n) {
var i;
var slides = document.getElementsByClassName("mySlides");
var dots = document.getElementsByClassName("dot");
if (n > slides.length) {slideIndex = 1}
if (n < 1) {slideIndex = slides.length}
for (i = 0; i < slides.length; i++) {
slides[i].style.display = "none";
}
for (i = 0; i < dots.length; i++) {
dots[i].className = dots[i].className.replace(" active","");
}
slides[slideIndex-1].style.display = "block";
dots[slideIndex-1].className += " active";
}
在我将其放入单独的脚本文件之前，一切似乎都工作正常。
<script src="js/slider.js"></script>
该功能 不能 可以在外部文件中编写的JavaScript上here使用。
` 这就是调试信息说的
Uncaught TypeError: Cannot read property 'style' of undefined
at showSlides (slider.js:27)
at slider.js:3`
showSlides(slideIndex); This shows error
slides[slideIndex-1].style.display = "block"; This shows error
解决方法
如果将<script>标签放置在<head>中，请检查它的位置，然后将其放置在</body>上方，这样可以解决您的问题。
请输入以下代码：
script.js
var slideIndex = 1;
showSlides(slideIndex);
function plusSlides(n) {
showSlides(slideIndex += n);
}
function currentSlide(n) {
showSlides(slideIndex = n);
}
function showSlides(n) {
var i;
var slides = document.getElementsByClassName("mySlides");
var dots = document.getElementsByClassName("dot");
if (n > slides.length) {
slideIndex = 1
}
if (n < 1) {
slideIndex = slides.length
}
for (i = 0; i < slides.length; i++) {
slides[i].style.display = "none";
}
for (i = 0; i < dots.length; i++) {
dots[i].className = dots[i].className.replace(" active","");
}
slides[slideIndex - 1].style.display = "block";
dots[slideIndex - 1].className += " active";
}
index.html
<!DOCTYPE html>
<html>
<head>
<title>Sample</title>
<style>
* {
box-sizing: border-box
}
body {
font-family: Verdana,sans-serif;
margin: 0
}
.mySlides {
display: none
}
img {
vertical-align: middle;
}
/* Slideshow container */
.slideshow-container {
max-width: 1000px;
position: relative;
margin: auto;
}
/* Next & previous buttons */
.prev,.next {
cursor: pointer;
position: absolute;
top: 50%;
width: auto;
padding: 16px;
margin-top: -22px;
color: white;
font-weight: bold;
font-size: 18px;
transition: 0.6s ease;
border-radius: 0 3px 3px 0;
user-select: none;
}
/* Position the "next button" to the right */
.next {
right: 0;
border-radius: 3px 0 0 3px;
}
/* On hover,add a black background color with a little bit see-through */
.prev:hover,.next:hover {
background-color: rgba(0,0.8);
}
/* Caption text */
.text {
color: #f2f2f2;
font-size: 15px;
padding: 8px 12px;
position: absolute;
bottom: 8px;
width: 100%;
text-align: center;
}
/* Number text (1/3 etc) */
.numbertext {
color: #f2f2f2;
font-size: 12px;
padding: 8px 12px;
position: absolute;
top: 0;
}
/* The dots/bullets/indicators */
.dot {
cursor: pointer;
height: 15px;
width: 15px;
margin: 0 2px;
background-color: #bbb;
border-radius: 50%;
display: inline-block;
transition: background-color 0.6s ease;
}
.active,.dot:hover {
background-color: #717171;
}
/* Fading animation */
.fade {
-webkit-animation-name: fade;
-webkit-animation-duration: 1.5s;
animation-name: fade;
animation-duration: 1.5s;
}
@-webkit-keyframes fade {
from {
opacity: .4
}
to {
opacity: 1
}
}
@keyframes fade {
from {
opacity: .4
}
to {
opacity: 1
}
}
/* On smaller screens,decrease text size */
@media only screen and (max-width: 300px) {
.prev,.next,.text {
font-size: 11px
}
}
</style>
</head>
<body>
<div class="slideshow-container">
<div class="mySlides fade">
<div class="numbertext">1 / 3</div>
<img src="img_nature_wide.jpg" style="width:100%">
<div class="text">Caption Text</div>
</div>
<div class="mySlides fade">
<div class="numbertext">2 / 3</div>
<img src="img_snow_wide.jpg" style="width:100%">
<div class="text">Caption Two</div>
</div>
<div class="mySlides fade">
<div class="numbertext">3 / 3</div>
<img src="img_mountains_wide.jpg" style="width:100%">
<div class="text">Caption Three</div>
</div>
<a class="prev" onclick="plusSlides(-1)">❮</a>
<a class="next" onclick="plusSlides(1)">❯</a>
</div>
<br>
<div style="text-align:center">
<span class="dot" onclick="currentSlide(1)"></span>
<span class="dot" onclick="currentSlide(2)"></span>
<span class="dot" onclick="currentSlide(3)"></span>
</div>
<script src="script.js"></script>
</body>
</html>
有关更多说明，请下载以下代码：Sample Slider.zip