JavaScript动态操作元素样式与类：实现可切换的UI状态

window.onload = function() {
  // 获取所有初始类名为 "button" 的元素
  const optionButtons = document.getElementsByClassName("button");
  // 获取“继续”按钮
  const forwardButton = document.getElementById("forward");

  // 遍历所有按钮，并为每个按钮添加点击事件监听器
  Array.from(optionButtons).forEach(function(button) {
    button.addEventListener("click", () => {
      // 获取当前点击按钮内部的 .key-selector 元素
      let keySelector = button.querySelector(".key-selector");

      // 判断当前按钮是否已处于选中状态
      if (button.classList.contains("selected-button")) {
        // 如果已选中，执行取消选中逻辑
        keySelector.style.color = "rgb(51, 51, 51)";
        keySelector.style.opacity = "0.6";

        button.style.backgroundColor = "rgb(226, 226, 226)";
        button.style.color = "black";
        button.style.opacity = "1";

        // 移除 "selected-button" 类
        button.classList.remove("selected-button");
        console.log("Deselected:", button.className);

        // TODO: 这里可以添加逻辑来检查是否还有其他按钮被选中，
        // 如果没有，则将 forwardButton 恢复到默认状态
      } else {
        // 如果未选中，执行选中逻辑
        button.style.backgroundColor = "rgb(77, 55, 120)";
        button.style.opacity = "0.65";
        button.style.color = "white";

        keySelector.style.color = "white";

        // 选中时，改变“继续”按钮的样式
        forwardButton.style.color = "white";
        forwardButton.style.backgroundColor = "rgb(77, 55, 120)";
        forwardButton.style.transition = "1s ease";

        // 添加 "selected-button" 类
        button.classList.add("selected-button");
        console.log("Selected:", button.className);
      }
    });
  });
};

/* 基础按钮样式 */
.button {
  background-color: rgb(226, 226, 226);
  /* ... 其他通用样式 ... */
  cursor: pointer;
  font-size: 18px;
  line-height: 16.8px;
  display: block;
  position: relative;
  border-radius: 5px;
  padding: 21px 25px 22px 25px;
  box-sizing: border-box;
}

/* 按钮悬停样式 */
.button:hover {
  background-color: rgb(194, 194, 194);
  opacity: 0.8;
}

/* 键选择器（内部元素）的基础样式 */
.key-selector {
  position: absolute;
  top: 50%;
  margin-top: -12px;
  font-size: 16px;
  line-height: 1.5em;
  text-align: center;
  width: 30px;
  display: block;
  opacity: 0.6;
  border: 1px solid;
  border-radius: 5px;
  height: 25px;
  color: rgb(51, 51, 51);
}

/* 按钮悬停时键选择器的样式 */
.button:hover .key-selector {
  color: black;
}

/*
  如果希望通过CSS来管理选中状态的样式，可以这样定义：
  .selected-button {
      background-color: rgb(77, 55, 120);
      opacity: 0.65;
      color: white;
  }
  .selected-button .key-selector {
      color: white;
  }
  然而，在提供的JS解决方案中，样式是直接通过 `element.style` 设置的，
  这会覆盖CSS规则，因此这些CSS规则可能不会生效，除非移除JS中的内联样式设置。
*/

/* 导航和底部按钮栏的样式 */
.navbar {
  display: flex;
  list-style: none;
  background-color: rgb(77, 55, 120);
  margin: 0;
  position: fixed;
  width: 100%;
  gap: 4rem;
  height: 50px;
  text-align: center;
  line-height: 45px;
  left: 0;
  top: 0;
}

.nav-text {
  text-decoration: none;
  color: white;
  width: auto;
  cursor: pointer;
  font-size: 18px;
  padding-bottom: 5px;
}

.instruction {
  padding-top: 8rem;
  left: 10%;
  padding-bottom: 0px;
  width: auto;
  max-width: 730px;
  font-size: 20px;
  position: absolute;
}

.options {
  height: auto;
  max-height: 313px;
  max-width: 750px;
  width: auto;
  padding-top: 15rem;
  padding-bottom: 60px;
  display: flex;
  flex-direction: column;
  gap: 15px;
  position: sticky;
  left: 8rem;
}

.text {
  margin-left: 4rem;
}

.button-bar {
  position: fixed;
  bottom: 0;
  width: 100%;
  display: flex;
  margin: 0;
  left: 0;
}

.nav-inner {
  cursor: pointer;
  width: 50%;
  text-align: center;
  line-height: 83px;
}

#backward {
  background-color: rgb(101, 93, 93);
  color: white;
}

#forward {
  background-color: rgb(191, 191, 191);
}

#backward:hover,
#forward:hover {
  background-color: rgb(77, 55, 120);
  color: white;
}

<div id="work-container">
  <ul class="navbar">
    <li class="section"><a class="nav-text" href="client.html">A</a></li>
    <li class="section"><a class="nav-text" href="case.html" style=" border-bottom: 1px solid;">B</a></li>
  </ul>
  <div class="instruction">
    <h3>Please choose an answer. You can choose more than one answers.</h3>
  </div>
  <div class="options">
    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>A</span>
        </div>
        <div class="text">1</div>
      </div>
    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>B</span>
        </div>
        <div class="text">2</div>
      </div>
    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>C</span>
        </div>
        <div class="text">3</div>
      </div>
    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>D</span>
        </div>
        <div class="text">4</div>
      </div>
    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>E</span>
        </div>
        <div class="text">5</div>
      </div>
    </div>
  </div>

  <div class="button-bar">
    <div class="nav-inner" id="backward">
      < Back</div>
        <div class="nav-inner" id="forward"> Continue ></div>
    </div>
  </div>
</div>