方法一：

利用input,textarea特有的select()方法，该元素不能设置display:none和 visibility: hidden;否则无效，可以设置opacity: 0;或者利用定位到屏幕之外不可见的地方

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <meta name='viewport' content='width=device-width, initial-scale=1.0, maximum-scale=1.0'/>
    <title></title>
    <style>
        body {
            padding: 20px;
            text-align: center;
        }
        .copytxt {
            opacity: 0; /*不可以用display:none和 visibility: hidden;*/
        }
    </style>
</head>
<body>
<a href="javascript:;" onclick="copyUrl()">点我复制</a>
<input type="text" class="copytxt"/>
<script>
    function copyUrl() {
        document.getElementsByClassName("copytxt")[0].value = "https://www.baidu.com";
        var url = document.getElementsByClassName("copytxt")[0];
        url.select(); // 选择需要复制内容的对象
        document.execCommand("Copy"); // 执行浏览器复制命令
        alert("复制成功");
    }
</script>

</body>
</html>

缺点：不兼容ios客户端，使用iphone手机复制无效

方法二：利用window对象的getSelection()方法，可兼容pc/ios/Android

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <meta name='viewport' content='width=device-width, initial-scale=1.0, maximum-scale=1.0' />
    <title></title>
    <style>
        body{
            padding: 20px;
            text-align: center;
        }
        .copytxt{
            opacity: 0;/*不可以用display:none和 visibility: hidden;*/
        }
    </style>
</head>
<body>
<a href="javascript:;" onclick="copyUrl()">点我复制</a>
<span class="copytxt"></span>
<script>
    function copyUrl(){
        document.getElementsByClassName("copytxt")[0].innerText = "https://www.baidu.com";
        const range = document.createRange();
        range.selectNode(document.querySelector(".copytxt"));
        const selection = window.getSelection();
        if(selection.rangeCount > 0) selection.removeAllRanges();
        selection.addRange(range);
        document.execCommand('Copy');
        alert("复制成功");
    }
</script>
</body>
</html>

原文作者技术博客：https://www.jianshu.com/u/ac4daaeecdfe