set_next(root, new_node) {
    if(root) {
        let current = root;
        while(current.next) {
            current = current.next;
        }
        current.next = new_node;
        return root;
    }
    else {
        return new_node;
    }
}

set_child_next(root, new_node) {
    if(root) {
        if(root.child) {
            return set_next(root, new_node);
        }
        else {
            root.child = new_node;
            return root;
        }
    }
    else {
        return new_node;
    }
}

let root = null;
root = set_next(root, new Node(...));

// or in case of we wanting to set: node.child, node.child.next, node.child.next.next, etc...
root = set_child_next(root, new Node(...));