javascript 製作複製按鈕

<p>点击复制后在右边textarea CTRL+V看一下</p>
<input type="text" id="inputText" value="测试文本"/>
<input type="button" id="btn" value="复制"/>
<textarea rows="4"></textarea>
<script type="text/javascript">
    var btn = document.getElementById('btn');
    btn.addEventListener('click', function(){
        var inputText = document.getElementById('inputText');
        var currentFocus = document.activeElement;
        inputText.focus();
        inputText.setSelectionRange(0, inputText.value.length);
        document.execCommand('copy', true);
        currentFocus.focus();
    });
</script>