JavaScript__之报错处理

报错提示文字

Uncaught TypeError: Cannot read property 'getElementsByTagName' of nulljavascript

报错控制台显示图片

html代码显示

<!DOCTYPE html>
<html lang="en">
<head>
    <link rel="stylesheet" type="text/css" href="../css/index.css">
    <!--这一种是在底部加载类容-->
    <script src="../js/index.js"></script>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div class="main" id="main">
    <!--图片轮播-->
    <div class="banner" id="banner">
        <a href="">
            <div class="banner-slide slide1 slide-active"></div>
        </a>
        <a href="">
            <div class="banner-slide slide2"></div>
        </a>
        <a href="">
            <div class="banner-slide slide3"></div>
        </a>
    </div>
    <!--上一张,下一章图片-->
    <a href="javascript:void(0);" class="button pre"></a>
    <a href="javascript:void(0);" class="button next"></a>
    <!--设置小圆点-->
    <div class="dots">
        <ul>
            <span class="active"></span>
            <span></span>
            <span></span>
        </ul>
    </div>
</div>
</body>
</html>
复制代码

解决过程,发现javaScript不该该写在head中,应该写在body中

更改后的代码css

<!DOCTYPE html>
<html lang="en">
<head>
    <link rel="stylesheet" type="text/css" href="../css/index.css">

    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div class="main" id="main">
    <!--图片轮播-->
    <div class="banner" id="banner">
        <a href="">
            <div class="banner-slide slide1 slide-active"></div>
        </a>
        <a href="">
            <div class="banner-slide slide2"></div>
        </a>
        <a href="">
            <div class="banner-slide slide3"></div>
        </a>
    </div>
    <!--上一张,下一章图片-->
    <a href="javascript:void(0);" class="button pre"></a>
    <a href="javascript:void(0);" class="button next"></a>
    <!--设置小圆点-->
    <div class="dots">
        <ul>
            <span class="active"></span>
            <span></span>
            <span></span>
        </ul>
    </div>
</div>
<!--这一种是在底部加载类容-->
<script src="../js/index.js"></script>
</body>
</html>
复制代码

最后运行结果

网页是从头部向下加载的,假如加载到头部的时候,这个时候尚未加载到div的时候就会报错
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