创建图片轮播遇TS错误：Property 'style'不存在于类型'Element'

问题场景 #

参照W3Schools教程实现图片自动轮播组件时，JavaScript代码在TypeScript环境中运行出现错误：Property 'style' does not exist on type 'Element'. ts(2339)，错误集中在.style.display相关逻辑。

原HTML代码 #

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<script src="script.js" ></script>

</head>
<body>

<h2>Automatic Slideshow</h2>
<p>Change image every 2 seconds:</p>

<div class="slideshow-container">

<div class="mySlides fade">
  <div class="numbertext">1 / 3</div>
  <img src="green.jpeg" alt="Trees"> 
  <div class="text">Caption Text</div>
</div>

<div class="mySlides fade">
  <div class="numbertext">2 / 3</div>
  <img src="map.jpeg" alt="Trees"> 
  <div class="text">Caption Two</div>
</div>

<div class="mySlides fade">
  <div class="numbertext">3 / 3</div>
  <img src="trees.jpeg" alt="Trees"> 
  <div class="text">Caption Three</div>
</div>

</div>
<br>

<div style="text-align:center">
  <span class="dot"></span> 
  <span class="dot"></span> 
  <span class="dot"></span> 
</div>
</body>
</html> 

原JavaScript代码（TS环境报错） #

let slideIndex = 0;
showSlides();

function showSlides() {
  let i;
  let slides = document.getElementsByClassName("mySlides");
  let dots = document.getElementsByClassName("dot");
  for (i = 0; i < slides.length; i++) {
    slides[i].style.display = "none";  
  }
  slideIndex++;
  if (slideIndex > slides.length) {slideIndex = 1}    
  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";
  setTimeout(showSlides, 2000); // Change image every 2 seconds
}

错误原因 #

document.getElementsByClassName()返回的是HTMLCollectionOf<Element>类型，TypeScript中Element基类并不包含style属性——style是HTMLElement的专属属性，因此直接访问会触发类型校验错误。

解决方案 #

以下是三种可行的修复方式：

方式1：类型断言为HTMLElement #

在访问style属性前，将元素断言为HTMLElement类型：

let slideIndex = 0;
showSlides();

function showSlides() {
  let i;
  let slides = document.getElementsByClassName("mySlides");
  let dots = document.getElementsByClassName("dot");
  for (i = 0; i < slides.length; i++) {
    (slides[i] as HTMLElement).style.display = "none";  
  }
  slideIndex++;
  if (slideIndex > slides.length) {slideIndex = 1}    
  for (i = 0; i < dots.length; i++) {
    (dots[i] as HTMLElement).className = (dots[i] as HTMLElement).className.replace(" active", "");
  }
  (slides[slideIndex-1] as HTMLElement).style.display = "block";  
  (dots[slideIndex-1] as HTMLElement).className += " active";
  setTimeout(showSlides, 2000);
}

方式2：使用querySelectorAll并指定类型 #

querySelectorAll支持泛型参数，可以直接指定返回元素类型为HTMLElement，避免重复断言：

let slideIndex = 0;
showSlides();

function showSlides() {
  let i;
  let slides = document.querySelectorAll<HTMLElement>(".mySlides");
  let dots = document.querySelectorAll<HTMLElement>(".dot");
  for (i = 0; i < slides.length; i++) {
    slides[i].style.display = "none";  
  }
  slideIndex++;
  if (slideIndex > slides.length) {slideIndex = 1}    
  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";
  setTimeout(showSlides, 2000);
}

方式3：给变量添加类型注解 #

直接给slides和dots变量指定HTMLCollectionOf<HTMLElement>类型：

let slideIndex = 0;
showSlides();

function showSlides() {
  let i;
  let slides: HTMLCollectionOf<HTMLElement> = document.getElementsByClassName("mySlides") as HTMLCollectionOf<HTMLElement>;
  let dots: HTMLCollectionOf<HTMLElement> = document.getElementsByClassName("dot") as HTMLCollectionOf<HTMLElement>;
  for (i = 0; i < slides.length; i++) {
    slides[i].style.display = "none";  
  }
  slideIndex++;
  if (slideIndex > slides.length) {slideIndex = 1}    
  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";
  setTimeout(showSlides, 2000);
}

内容的提问来源于stack exchange，提问作者Mohamed Aly