html

<p>单击按钮以复制输入框中的文本，然后尝试将文本（例如 ctrl+v）粘贴到不同的窗口中，以查看效果。</p>

<input type="text" value="RUNOOB" id="myInput">

<div class="tooltip">
<button onclick="myFunction()" onmouseout="outFunc()">
  <span class="tooltiptext" id="myTooltip">复制</span>
  复制内容提示
  </button>
</div>
<p>
可以粘贴到下面：
</p>
<textarea></textarea>

js

function myFunction() {
  var copyText = document.getElementById("myInput");
  copyText.select();
  copyText.setSelectionRange(0, 99999);
  navigator.clipboard.writeText(copyText.value);
  
  var tooltip = document.getElementById("myTooltip");
  tooltip.innerHTML = "复制内容: " + copyText.value;
}

function outFunc() {
  var tooltip = document.getElementById("myTooltip");
  tooltip.innerHTML = "点击按钮复制";
}

css

.tooltip {
  position: relative;
  display: inline-block;
}

.tooltip .tooltiptext {
  visibility: hidden;
  width: 140px;
  background-color: #555;
  color: #fff;
  text-align: center;
  border-radius: 6px;
  padding: 5px;
  position: absolute;
  z-index: 1;
  bottom: 150%;
  left: 50%;
  margin-left: -75px;
  opacity: 0;
  transition: opacity 0.3s;
}

.tooltip .tooltiptext::after {
  content: "";
  position: absolute;
  top: 100%;
  left: 50%;
  margin-left: -5px;
  border-width: 5px;
  border-style: solid;
  border-color: #555 transparent transparent transparent;
}

.tooltip:hover .tooltiptext {
  visibility: visible;
  opacity: 1;
}

转载
JS 复制文本 | 菜鸟工具
https://c.runoob.com/codedemo/7685/