problem moving character between multiple nodes, storing next target and previous target in array and moving between them (JS)

ive been trying to create a script that stores multiple nodes in a list as transforms and allows a player to move between them using the A and D keys, it works fine going through the array forwards with D but i cant seem to get it to move through the array whilst storing the current node and previous node so it moves between them. im pretty new to coding and a lot of this is adapted through various searches ive done trying to solve the problem. heres what i have -

 import System.Collections.Generic;
 import System.Linq;
 
 var wayPoints : List.<Transform>;
 var currentTarget : Transform; 
 var speed : float = 4000;
 var turnSpeed = 40;
 
 function Awake ()
 {
 // creates node array, adds nodes tagged "Node", sets up initial target node
 
     wayPoints = new List.<Transform>();
     AddNodes();
     TargetNode();
 }
 
 function Update () 
 {
 
 // creates target vector to rotate towards on key press
         
     var currentTargetDir = currentTarget.position - transform.position;
     var step = turnSpeed * Time.deltaTime;
     var currentDir = Vector3.RotateTowards(transform.forward, currentTargetDir, step, 0.0);;
 
     if (Input.GetKey("d"))
     {
         transform.rotation = Quaternion.LookRotation(currentDir);
         rigidbody.AddRelativeForce (Vector3.forward * speed * Time.deltaTime);
     }
 
 //    here i want to have it so pressing A rotates the player to the previous node and moves towards it and then continues
 //  going backwards through the array until D is pressed and then it moves forwards though array again.
 //
 //    if (Input.GetKey("a"))
 //    {
 //        transform.rotation = Quaternion.LookRotation(previousTargetDir);
 //        rigidbody.AddRelativeForce (Vector3.forward * speed * Time.deltaTime);
 //    }
 
 //  when player is 2 units from the target node, stop velocity and change target to next in the array
 //
     
     var distance : float = Vector3.Distance(currentTarget.transform.position, transform.position);
         
     if(distance < 2f)
     {
         rigidbody.velocity.x = 0;
         rigidbody.velocity.z = 0;
         TargetNode();
     }
 } 
 
 function AddNodes()
 {
     var nodeArray : GameObject[] = GameObject.FindGameObjectsWithTag("Node");
     
     for (var node : GameObject in nodeArray)
     {
         AddTarget(node.transform);
     }
 }
 
 function AddTarget (node : Transform)
 {
     wayPoints.Add(node);
     wayPoints = wayPoints.OrderBy(function(x){return x.name;}).ToList();
 }
 
 // have tried to store previousTarget by keeping it one step behind next but it only goes back to the last node and then
 // stops
 
 function TargetNode()
 {
     if(currentTarget == null)
     {
         currentTarget = wayPoints[0];
     }
     else
     {
         var next : int = wayPoints.IndexOf(currentTarget);
     }
     if(next < wayPoints.Count - 1)
     {
         next++;
     }
     else
     {
         next = 0;
     }
     
     currentTarget = wayPoints[next];
 }

some things ive tried -

• creating a variable alongside next in TargetNode() that stays one step behind it but it only returns to previous not back through array

• using next++ or next-- based on diection its moving

• created two seperate TargetNode() based on direction and/or proximity to previoustarget or current target, one ++ one --

• duplicating the list and reversing it, linking previous target var to this.

sorry for a long post but ive been going over and over this and i cant get it to acheive the node based movement i want, any suggestions or advice would be appreciated, like i said im new to coding.. please no suggesting iTween or splinecontroller or any anything, ive been through them and it doesnt work how i want it to, thanks in advance.