传统的表单控件textarea元素不支持通过javascript设置readonly属性示例是什么？

<input id="btn1" type="button" value="禁用">
<input id="btn2" type="button" value="启用">
<input id="btn3" type="button" value="只读">
<input id="btn4" type="button" value="读写">
<textarea id="test">测试内容</textarea>
<script>
btn1.onclick = function(){
    test.setAttribute('disabled','');
};
btn2.onclick = function(){
    test.removeAttribute('disabled');
};    
btn3.onclick = function(){
    test.setAttribute('readonly','readonly');
};
btn4.onclick = function(){
    test.removeAttribute('readonly');
};
</script>