HTML代码 

<div hidden id="content">待复制的内容</div>

<btn class="btn btn-sm btn-default" onclick="copyBtn()">点此复制</btn>

JavaScript 代码

function copyBtn() {
var content= document.getElementById("content").innerText;
var oInput = document.createElement('input');
oInput.value = content;
document.body.appendChild(oInput);
oInput.select();
document.execCommand("Copy");
oInput.className = 'oInput';
oInput.style.display = 'none';
alert('复制成功');
}