Find the Preorder Successor of a Given Node in a Binary Tree using JavaScript

class Vertex {
    constructor(key) {
        this.key = key;
        this.left = this.right = this.parent = null;
    }
}

function presuccessor(root, v) {
    if (v.left) return v.left;
    if (v.right) return v.right;

    let current = v,
        parent = current.parent;
    while (parent && parent.right === current) {
        current = parent;
        parent = parent.parent;
    }
    return parent ? parent.right : null;
}

// Create the binary tree
let root = new Vertex(1);
root.left = new Vertex(2);
root.left.left = new Vertex(4);
root.left.right = new Vertex(5);
root.right = new Vertex(3);
root.right.left = new Vertex(6);
root.right.right = new Vertex(7);

// Set parent pointers
root.left.parent = root;
root.left.left.parent = root.left;
root.left.right.parent = root.left;
root.right.parent = root;
root.right.left.parent = root.right;
root.right.right.parent = root.right;

// Find the preorder successor and print the result
let node = root.left.right;
let successor = presuccessor(root, node);
if (successor) {
    console.log("Preorder Successor of " + node.key + " is " + successor.key);
} else {
    console.log("Preorder successor of " + node.key + " is null");
}