Mutation in javascript?

Question

In the below code size2() method working fine.But in size1() it is mutating the object and making it null.why such behavior is not happening in size2()?

class Node {
    constructor(data, next = null) {
        this.data = data;
        this.next = next;
    }
}

class LinkedList {
    constructor() {
        this.head = null;
    }

    insert(data) {
        this.head = new Node(data, this.head);
    }

    size1() {
        var counter = 0;
        while (this.head) {
            counter++;
            this.head = this.head.next;
        }
        return counter;
    }

    size2() {
        var counter = 0;
        var node = this.head;
        while (node) {
            counter++;
            node = node.next;
        }
        return counter;
    }
}
var list = new LinkedList();
list.insert(35);

console.log(list);
console.log(list.size2());

To me both the method looks same. Is there any subtle difference in those methods?

Answer 1

In size2(), you are not mutating this.head because you first copy the reference in a local variable. Since in the while loop you are mutating the local node = node.next starting here nodeand this.head are not linked any more. It's the eternal Value/Reference pitfall.

Here is a related article.