Javascript – Why I'm hitting time limit!? LeetCode Linked List Cycle (Solved but explanation needed!)

I’m working on solving this problem on leetcode (https://leetcode.com/problems/linked-list-cycle/description/) and my original solution exceeded the time limit (Example A). I was eventually able to make the solution fast enough by changing two lines of code and the solution was accepted and some how more performant. Why is example B faster than example A? Why is the variable assignment faster/more performant than using the original object?

I changed this…

Example A (Failing Solution)

var hasCycle = function(head) {
// some code ...
   let fastPointer = null;

   fastPointer = head?.next?.next || null;
// ...
}

to this …

Example B (Working Solution)

var hasCycle = function(head) {
// some code ...
   let fastPointer = head;

   fastPointer = fastPointer?.next?.next || null;
// ...
}

Here is the full code for both solutions…

Example A (Failing Solution)….

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
   // create a fast pointer
   let fastPointer = null;

   // create a while loop that lasts for as long as head exists

   while (head) {
    // move fast pointer over by 2
    fastPointer = head?.next?.next || null;

    // if fastpointer is null, this indicates that there is no cycle so return false
    if (fastPointer === null) return false;

    // compare fastPointer to the slow pointer, if they are ever equal to the same node then the list contains a cycle.
    if (fastPointer === head) return true;

    // move head by 1
    head = head.next;


   }




   // if the while loop ends, it indicates the there is no cycle so return false

   return false;
};   

Example B (Working Solution)…

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
   // create a fast pointer
   let fastPointer = head;

   // create a while loop that lasts for as long as head exists

   while (head) {
    // move fast pointer over by 2 
    // !!!CHANGE HAPPENED HERE and solution worked!!!!
    fastPointer = fastPointer?.next?.next || null;

    // if fastpointer is null, this indicates that there is no cycle so return false
    if (fastPointer === null) return false;

    // compare fastPointer to the slow pointer, if they are ever equal to the same node then the list contains a cycle.
    if (fastPointer === head) return true;

    // move head by 1
    head = head.next;


   }


   // if the while loop ends, it indicates the there is no cycle so return false

   return false;
};