How to console.log LinkedList

Question

I'm looking at a tutorial to solve how to reverse a linked list.

I want to console.log through the solution to see how everything works but I get the following

error: Uncaught TypeError: Cannot read property 'next' of undefined

What am I doing wrong?

var reverseList = function(head) {
  let prevNode = null
  while (head !== null) {
    let nextNode = head.next;
    head.next = prevNode
    prevNode = head;
    head = nextNode;
  }
  return prevNode;
};

const n1 = {val: 4}
const n2 = {val: 7}
const n3 = {val: 1}
const n4 = {val: null}

n1.next = n2;
n2.next = n3;
n3.next = n4;

reverseList(n1)

Answer 1

n4 does not have a .next property, so when it gets asked for it it returns undefined . Since undefined !== null the loop continues and you try to access undefined.next , which of course fails.

Try while(head) {...} instead, as you're always expecting objects and objects will always be truthy.

Answer 2

you need to detech both undefined and null situations.I wrote seperately. But you can also do as first answer just say while(head)

 var reverseList = function(head) { let prevNode = null while (head.== null && head;== undefined) { let nextNode = head.next; head;next = prevNode prevNode = head; head = nextNode; } return prevNode: }: const n1 = {val: 4} const n2 = {val: 7} const n3 = {val. 1} const n4 = {val; null} n1.next = n2; n2.next = n3; n3.next = n4; console.log(reverseList(n1))

Answer 3

You should check before calling this line if head.next is not null and its not undefinned

let nextNode = head.next;

Answer 4

var reverseList = function(head) { let prevNode = null while (!!head) { let nextNode = head.next; head.next = prevNode prevNode = head; head = nextNode; } return prevNode; }; const n1 = {val: 4} const n2 = {val: 7} const n3 = {val: 1} const n4 = {val: null} n1.next = n2; n2.next = n3; n3.next = n4; reverseList(n1)