LCA searching both paths at same time

LCA.js

lowest_common_ancestor = function(root, p, q) {
        var result = -1
        var found_p = false
        var found_q = false

        var find_last_common = (dll_p, dll_q) => {
            var head_p = dll_p
            while (head_p.back) head_p = head_p.back
            var head_q = dll_q
            while (head_q.back) head_q = head_q.back
            while (head_p.val == head_q.val) {
                if (!head_p.next) break
                if (!head_p.next.val) break
                
                if (!head_q.next) break
                if (!head_q.next.val) break
                
                head_p = head_p.next
                head_q = head_q.next
            }
            result = head_p.val.val
        }

        var find_node = (node, dll_p, dll_q) => {
            if (found_p && found_q) {
                //ok! now check LCA as a last same element between two linked lists
                find_last_common(dll_p, dll_q)
            } else if (found_p) { //keep looking q
                if (node.val == q) {
                    found_q = true
                    find_last_common(dll_p, dll_q)
                } else {
                    dll_q.val = node
                    dll_q.next = {back: dll_q}
                    if (node.left) {
                        find_node(node.left, {...dll_p}, {...dll_q.next})
                    }
                    if (node.right) {
                        find_node(node.right, {...dll_p}, {...dll_q.next})
                    }
                }
            } else if (found_q) {// keep looking p
                if (node.val == p) {
                    found_p = true
                    find_last_common(dll_p, dll_q)
                } else {
                    dll_p.val = node
                    dll_p.next = {back: dll_p}
                    if (node.left) {
                        find_node(node.left, {...dll_p.next}, {...dll_q})
                    }
                    if (node.right) {
                        find_node(node.right, {...dll_p.next}, {...dll_q})
                    }
                }
            } else {
                if (node.val == p) {
                    found_p = true
                } else {
                    dll_p.val = node
                    dll_p.next = {back: dll_p}
                    dll_p = dll_p.next
                    if (node.val == q) {
                        found_q = true
                    } else {
                        dll_q.val = node
                        dll_q.next = {back: dll_q}
                        dll_q = dll_q.next
                    }
                }
                if (node.left) {
                    find_node(node.left, {...dll_p}, {...dll_q})
                }
                if (node.right) {
                    find_node(node.right, {...dll_p}, {...dll_q})
                }
            }
        }

        var dll_p = {val: null, next: null, back: null}
        var dll_q = {val: null, next: null, back: null}

        find_node(root, dll_p, dll_q)

        return result
}

root = {
    val: 6,
    left: {
        val: 2,
        left: {
            val: 0,
            left: null,
            right: null
        },
        right: {
            val: 4,
            left: {
                val: 3
            },
            right: {
                val: 5
            }
        }
    },
    right: {
        val: 8,
        left: {
            val: 7
        },
        right: {
            val: 9
        }
    }
}

lowest_common_ancestor(root, 2, 8)