Modifications not propagating through object reference

Here since the pointers fast and slow are referencing the same object dummy, why do the modificaitons in slow donot modify the fast pointer?

    function ListNode(val, next) {
     this.val = (val===undefined ? 0 : val)
     this.next = (next===undefined ? null : next)
 }

 var removeNthFromEnd = function(head, n) {
    let dummy = new ListNode(0);
    dummy.next = head;

    let slow = dummy;
    let fast = dummy;
    slow = slow.next
    console.log(fast)


    for (let i = 0; i <= n; i++) {
        fast = fast.next;
    }

    while (fast !== null) {
        slow = slow.next;
        fast = fast.next;
    }


    return dummy.next;
};
removeNthFromEnd( {val: 1, next:{val:2, next:{val:3, next: {val: 4,next:{val:5,next:null}}}}},2)

This type of behaviour does not occur when i directly assign object to variables