test.html

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>原生 js 实现点击按钮复制文本</title>
        <style type="text/css">
        .wrapper {position: relative;}
        #input {position: absolute;top: 0;left: 0;opacity: 0;z-index: -10;}
        </style>

        <script type="text/javascript">
            function copyText() {
                var text = document.getElementById("text").innerHTML;
                var input = document.getElementById("input");
                input.value = text;
                input.select();//选中文本
                document.execCommand("copy");
                alert("Copied!");
            }
        </script>
    </head>
    <body>
        <div class="wrapper">
        <a id="text" onclick="copyText()">123</a>
        <textarea id="input">这是幕后黑手</textarea>
        </div>
    </body>
</html>