Add Two Numbers II LeetCode solution in JavaScript

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var addTwoNumbers = function (l1, l2) {
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  const reverse = (head) => {
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    let prev = null;
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    let dummy = head;
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    while (dummy) {
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      let temp = dummy.next;
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      dummy.next = prev;
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      prev = dummy;
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      dummy = temp;
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    }
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    return prev;
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  };
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  let head1 = reverse(l1);
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  let head2 = reverse(l2);
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  // add
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  let sum = new ListNode();
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  let p = sum;
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  let carry = 0;
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  while ((head1 && head2) || carry) {
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    let v1 = head1 ? head1.val : 0;
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    let v2 = head2 ? head2.val : 0;
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    let v = v1 + v2 + carry;
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    if (v >= 10) carry = 1;
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    else carry = 0;
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    let node = new ListNode(v % 10);
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    sum.next = node;
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    sum = sum.next;
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    head1 = head1 && head1.next;
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    head2 = head2 && head2.next;
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  }
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  sum.next = head1 || head2;
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  return reverse(p.next);
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};