js的shift()方法失效?

Question

如题,代码如下:

<ul class="demo">

    <li>1</li>

    <li>2</li>

    <li>3</li>

    <li>4</li>

    <li>5</li>

    <li>6</li>

</ul>

<script>

    var l = document.getElementsByClassName("demo")[0];

    var a = l.children;

    var r=a.shift();

    console.log(r);//报错:a.shift is not a function?

</script>

类数组对象不能调用shift方法api?