Add Two Numbers LeetCode solution in JavaScript

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/**
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 * Definition for singly-linked list.
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 * function ListNode(val) {
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 *     this.val = val;
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 *     this.next = null;
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 * }
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 */
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/**
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 * @param {ListNode} l1
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 * @param {ListNode} l2
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 * @return {ListNode}
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 */
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var addTwoNumbers = function (l1, l2) {
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  var List = new ListNode(0);
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  var head = List;
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  var sum = 0;
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  var carry = 0;
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  while (l1 !== null || l2 !== null || sum > 0) {
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    if (l1 !== null) {
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      sum = sum + l1.val;
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      l1 = l1.next;
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    }
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    if (l2 !== null) {
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      sum = sum + l2.val;
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      l2 = l2.next;
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    }
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    if (sum >= 10) {
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      carry = 1;
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      sum = sum - 10;
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    }
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    head.next = new ListNode(sum);
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    head = head.next;
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    sum = carry;
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    carry = 0;
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  }
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  return List.next;
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};