addTwoHugeNumbers.js

// Definition for singly-linked list:
// function ListNode(x) {
//   this.value = x;
//   this.next = null;
// }
//

function getNumber(list) {
  let current = list;
  let num = [];
  while (current !== null) {
    num.push(current.value);
    current = current.next;
  }

  return num;
}

function addTwoHugeNumbers(a, b) {
  const A = getNumber(a).reverse();
  const B = getNumber(b).reverse();
  const largest = A.length > B.length ? A : B;
  let sum = 0;
  let carry = 0;
  const result = [];
  largest.forEach((num, index) => {    
    const smallest = A.length > B.length ? B : A;
    const adder = smallest[index] || 0;
    sum = num + adder;
    if (carry) {
      sum += +carry;
    }
    const sumLen = sum.toString().length;
    if (sumLen > 4) {
      const carryLen = sumLen - 4;
      carry = +sum.toString().slice(0, carryLen);
      const thousands = +sum.toString().slice(carry, sum.length);
      result.push(thousands ? thousands : 0);
    } else {
      carry = 0;
      if (sum) {
        result.push(sum);
      } else {
        result.push(largest[index]);
      }
    }
  });
  
  if (carry) {
    result.push(carry);
  }
  
  return result.reverse();
}