原生JS给元素添加class属性

有下面这三种简单语句。

　　　　document.getElementsByTagName('body')[0].className = 'snow-container'; //设置为新的

        document.getElementsByTagName('body')[0].className += 'snow-container'; //在原来的后面加这个

        document.getElementsByTagName('body')[0].classList.add("snow-container"); //与第一个等价

测试一:

<!DOCTYPE html>

<html>

    <head>

        <meta charset="UTF-8">

        <title></title>

        <style type="text/css">

            .snow-container {

                background-color: red;

            }

        </style>

        <script type="text/javascript">

            window.onload = function() {

                document.getElementsByTagName('body')[0].className = 'snow-container'; //设置为新的

                document.getElementsByTagName('body')[0].className += 'snow-container'; //在原来的后面加这个

                document.getElementsByTagName('body')[0].classList.add("snow-container"); //与第一个等价

            }

        </script>

    </head>

    <body class="test">

    </body>

</html>

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" alt="" />

测试二:

<!DOCTYPE html>

<html>

    <head>

        <meta charset="UTF-8">

        <title></title>

        <style type="text/css">

            .snow-container {

                background-color: red;

            }

        </style>

    </head>

    <body class="test">

    </body>

    <script type="text/javascript">

        document.getElementsByTagName('body')[0].className = 'snow-container'; //设置为新的

        document.getElementsByTagName('body')[0].className += 'snow-container'; //在原来的后面加这个

        document.getElementsByTagName('body')[0].classList.add("snow-container"); //与第一个等价

    </script>

</html>

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" alt="" />

小结:JS放在body与head中的不同

放在body和head其实差不多的，只不过是文档解析的时间不同。浏览器解析html是从上到下的。
如果把javascript放在head里的话，则先被解析,但这时候body还没有解析，所以$(#btn)会返回空值。如果把javascript放在head里的话，一般都会绑定一个监听，当全部的html文档解析完之后，再执行代码：

$(document).ready(function(){
//这里放入执行代码
})
一般情况下最好是单独把javascript放在js文件里，通过head里的<script
src="file.js"></script>链接起来，css则是通过<link>。这样做的目的是为了让内容-样式-逻辑分离，以便以后能够更好地维护。

巴特西

原生JS给元素添加class属性

小结:JS放在body与head中的不同

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