为什么我的按钮与onclick事件不同步？

我当时正在学习JavaScript，并观看了有关使用onclick事件的视频，{https://www.youtube.com/watch?v=XQEfWd1lh4Q&list=PL4cUxeGkcC9i9Ae2D9Ee1RvylH38dKuET&index=41&t=23s).Basically，这个人制作了一张卡片，可以扩展以显示其截取的部分内容。只是通过观看视频而有所了解。因此，为了加深我的理解，我决定重新编码视频中显示的内容，而无需回头。

我有点想要的东西，但是有一件小事让我感到沮丧。运行代码时，我的浓缩卡带有显示“ Show More”（显示更多）的按钮（不错），但是当我按下按钮时，卡会展开，但按钮仍显示“ Show More”（显示更多）它开始变坏），并且只有当我第三次按下按钮来压缩卡时，按钮才会显示“ Show Less”。

var contentDiv = document.querySelector('.content');
var button = document.querySelector('.button');

button.onclick = function(){
  if(contentDiv.classname == 'content'){
    contentDiv.classList.add('appear');
    button.innerHTML = 'Show More';
  } else if(contentDiv.classname == 'content appear'){
    contentDiv.classList.remove('appear');
    button.innerHTML = 'Show Less';
  }
};

*{
  margin: 0;
  padding: 0;
}

body{
  background: #a1a1a1;
}

.group{
  margin-top: 10%;
  display: flex;
  flex-flow: column nowrap;
  align-items: center;
}

.content{
  background: #dedede;
  border-radius: 10px;
  width: 250px;
  height: 300px;
  overflow: hidden;
  color: #2e2e2e;
  background: #e8e8e8;

  transition: all .5s;
}

.appear{
  height: 530px;
  color: #e8e8e8;
  background: #2e2e2e;
}

.content p{
  margin: 10px;
}

.button{
  margin: 10px;
  background: #f0ca62;
  padding: 5px;
  border-radius: 10px;
  cursor: pointer;
}

<!DOCTYPE html>
<html>
  <head>
    <title>Events</title>
    <link rel="stylesheet" href="./css/style.css">
  </head>
  <body>
    <div class="group">
      <div class="content">
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          width inflation scrape lamp continuation insistence proof pioneer trait
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          width inflation scrape lamp continuation insistence proof pioneer trait
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        </p>
        <p>width inflation scrape lamp continuation insistence proof pioneer trait
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          width inflation scrape lamp continuation insistence proof pioneer trait
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        </p>
        <p>width inflation scrape lamp continuation insistence proof pioneer trait
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        </p>
      </div>

      <a class="button">Show More</a>
    </div>


    <script type="text/javascript" src="./js/script.js"></script>
  </body>
</html>

link to code in codepen

为什么我的按钮与onclick事件不同步？

leehappygirl 回答：为什么我的按钮与onclick事件不同步？

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