您的内部 for 循环具有一个 i 变量，该变量与具有相同名称的外部 for 循环变量冲突。

您还应该在将单击的元素设置为“selected”之前从所有元素中删除 selected 类。

尝试：

<script>
    var tabs = document.getElementById("tabs").getElementsByTagName("li");
    var panels = document.getElementById("panels").getElementsByTagName("div");
    for (var i = 0; i < tabs.length; i++) {
        new function(i) {
            tabs[i].onclick = function() {
                for (var j = 0; j < panels.length; j++) {
                    tabs[j].className = panels[j].className = "";
                }
                tabs[i].className = panels[i].className = "selected";
            }
        }(i);
    }
</script>

您遇到了几个问题：

• 多个 i 变量
• new function(i) {... 不是最佳语法。我在下面使用了闭包
• 每行多次赋值并不好

我已为您的 <li> 元素赋值，以便我们可以判断哪个 li 元素已被点击

var tabs = document.getElementById("tabs").getElementsByTagName("li");
    var panels = document.getElementById("panels").getElementsByTagName("div");
    for (var i = 0; i < panels.length; i++) {
      (function(i) {
        tabs[i].onclick = function() {
          var j;
          var panelIndex;
          
          // remove styles from other tabs
          for (j = 0; j < tabs.length; j++) {
            tabs[j].className = "";
          }
          
          // apply style to the current tab: 'this'
          this.className = "selected";
          

          // hide other panels
          for (j = 0; j < panels.length; j++) {
            panels[j].className = "";
          }
          
          // show the selected panel
          panelIndex = +this.value;  // convert value to number
          
          panels[panelIndex-1].className="selected"; // arrays are 0-indexed, so subtract 1

        }
      })(i);
    }

a { text-decoration: none; }
li { list-style: none; }
li.selected { font-weight: bold; } 
.panels div { display: none; }
.panels .selected { display: block; }

<div id="tabs" class="tabs">
    <ul>
        <li value="1" class="selected"><a href="javascript:;">One</a></li>
        <li value="2" class=""><a href="javascript:;">Two</a></li>
        <li value="3" class=""><a href="javascript:;">Three</a></li>
    </ul>
</div>

<div id="panels" class="panels">
    <div class="selected">This is panel one.</div>
    <div class="">This is panel two.</div>
    <div class="">This is panel three.</div>
</div>

下面将按预期工作。我发现在 for 循环内访问 HTML 元素有两个问题，您需要使用 .item() 作为要获取的 HTMLCollection，而不是数组。此外，您的内部 for 循环需要使用不同的循环索引，并附加一个 if 条件，使单击的元素保持显示状态，其余元素隐藏。

<style>
    a { text-decoration: none; }
    li { list-style: none; }
    li.selected { font-weight: bold; } 
    .panels div { display: none; }
    .panels .selected { display: block; }
</style>

<div id="tabs" class="tabs">
    <ul>
        <li class="selected"><a href="javascript:;">One</a></li>
        <li class=""><a href="javascript:;">Two</a></li>
        <li class=""><a href="javascript:;">Three</a></li>
    </ul>
</div>

<div id="panels" class="panels">
    <div class="selected">This is panel one.</div>
    <div class="">This is panel two.</div>
    <div class="">This is panel three.</div>
</div>

<script>
    var tabs = document.getElementById("tabs").getElementsByTagName("li");
    var panels = document.getElementById("panels").getElementsByTagName("div");
    for (var i = 0; i < tabs.length; i++) {
        new function(i) {
            tabs[i].onclick = function() {
                tabs.item(i).className = panels.item(i).className = "selected";
                for (var j = 0; j < panels.length; j++) {
                    if(i!=j){
                        tabs.item(j).className = panels.item(j).className = "";
                    }
                }
            }
        }(i);
    }
</script>