Functional/ Variable scope in a javascript linked list implementaion-CodePudding

Reference to javascript solution of leetcode - Add two numbers https://leetcode.com/problems/add-two-numbers/

Why List scope is not changed when head scope is changed i.e. when head.next is assigned to new node reference and head is assigned back to head.next, then why List.next didn't change and remained the same as the whole linked list

Pls refer to below solution -

Definition for singly-linked list.
function ListNode(val) {
    this.val = val;
    this.next = null;
}
@param {ListNode} l1
@param {ListNode} l2
@return {ListNode}

var addTwoNumbers = function(l1, l2) {
    var List = new ListNode(0);
    var head = List;
    var sum = 0;
    var carry = 0;

    while(l1!==null||l2!==null||sum>0){

        if(l1!==null){
            sum = sum   l1.val;
            l1 = l1.next;
        }
        if(l2!==null){
            sum = sum   l2.val;
            l2 = l2.next;
        }
        if(sum>=10){
            carry = 1;
            sum = sum - 10;
        }

        head.next = new ListNode(sum);
        head = head.next;

        sum = carry;
        carry = 0;

    }

    return List.next;
};

I tried the below thing but it gave some different output below, b.next is changed when a.next is changed, Why so?

function value (val){ this.x = val;this.next = null;}
let a = new value(1);
console.log(a.x);
console.log(a.next, "a next");
let b = a;
console.log(b.x);
console.log(b.next,"b next")
a.next = 23;
console.log(a.next, "a next");
console.log(b.next,"b next")

VM1592:3 1
VM1592:4 null 'a next'
VM1592:6 1
VM1592:7 null 'b next'
VM1592:9 23 'a next'
VM1592:10 23 'b next'

List is not changed in above example but head did change? Why b is changed with a changed ? Why

CodePudding user response：

head and List are variables that hold references to the same ListNode. A ListNode is a mutable object. Therefore, when you change the properties of head, say change its next property, this is reflected also in List`, since they both point to the same object.

Now, when you change the value of head, say head = head.next, List still refers to the initial ListNode. head and List aren't references of each other. Changing one itself doesn't change the other. They are just pointers to an object. Only changing the properties of the object itself is reflected in the other, so long as both point to the same object (at the beginning that is).