getElementsByClassName 返回实时 HTMLCollection 对象，从元素中删除类时会更新。

getElementsByClassName returns a live HTMLCollection object, which gets updated when you remove the class from the elements.

因此，您可以使用的一个简单逻辑是使用while循环并查看是否该集合包含任何项目，如果是这样，则从第一项中删除该类。

So one simple logic you can use is to use a while loop and see if the collection has any items, if so remove the class from the first item.

function removeClass() {
  var elements = document.getElementsByClassName("test");
  console.log("Length " + elements.length)
  while (elements.length) {
    elements[0].setAttribute("class", "");
  }
}

.test {
  color: red;
}

<span class="test" id="test1">Test1 </span>
<span class="test" id="test2">Test2 </span>
<span class="test" id="test3">Test3 </span>
<button onclick="removeClass()">Test</button>