The class array is not an array, and there is no related API that inherits the array.
You can use call or apply to bind this,
For example

var r = [].shift.call(a)

ps: shift also involves operating the contents of the array. I just tried it and forced call to shift the array object. It will report that the length of the related object cannot be modified. If it also involves DOM processing, it is recommended to use related DOM operations. For example, removeChild will not be expanded. Relevant information about DOM array objects can be found in mdn, such as: https://developer.mozilla.org...

Shift will modify the original array, causing the length property to change, but length is read-only. It can be used in the following ways.

<ul class="demo">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
</ul>
</body>

<script>
    var l = document.getElementsByClassName("demo")[0];
    var a = l.children;
    var arry = [...a];
    var r=arry.shift();
    console.log(r);
</script>

Of course, shift is an array method. You can convert the class array into an array first and then call it
Array.prototype.slice.call(arraylike);

console.log(a)
You can see: __proto__:HTMLCollection There is no shift method in HTMLCollection.