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?