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https://www.imooc.com/wenda/detail/335116

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https://www.imooc.com/wenda/detail/343928

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https://www.imooc.com/wenda/detail/355889

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https://www.imooc.com/wenda/detail/354199

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https://www.imooc.com/wenda/detail/351029

https://www.imooc.com/wenda/detail/338104

https://www.imooc.com/wenda/detail/341333

https://www.imooc.com/wenda/detail/349765

https://www.imooc.com/wenda/detail/340295

https://www.imooc.com/wenda/detail/344260

https://www.imooc.com/wenda/detail/347652

https://www.imooc.com/wenda/detail/333079

https://www.imooc.com/wenda/detail/347332

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https://www.imooc.com/wenda/detail/332846

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https://www.imooc.com/wenda/detail/348389

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https://www.imooc.com/wenda/detail/340295

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https://www.imooc.com/wenda/detail/347652

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https://www.imooc.com/wenda/detail/340565

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https://www.imooc.com/wenda/detail/351029

第17行脚本看下，应该是没有定义对象变量，导致无法设置其对象的属性，null不可能有属性，或者是属性被定义成不可设置值为null

你的id拼写有误，在第二张图片中显示，在你的行内代码中，id=“mian”，但是在第一张js代码中，你用的byId（"main"），这样是获取不到元素的，所以出错

15行 

var main = byId("#main")