Level Order Tree Traversal

function levelOrder(root) {

    let queue = [root], result = [];
    function process(node) {
        result.push(node.val);
    }

    while (queue.length) {
        const node = queue.pop();
        process(node);
        if (node.left) queue.unshift(node.left);
        if (node.right) queue.unshift(node.right);
    }

    return result; //?
}