Why does my variable return null instead of the defined value

var addBinary = function(a, b) {
  let i = a.length - 1
  let j = b.length - 1

  let result = []
  var carry = 0;
  while (i >= 0 || j >= 0) {
    if (i == 0) {
      console.log(carry)
    }

    let sum = carry

    if (i >= 0) {
      sum = sum + parseInt(a[i])
    }

    if (j >= 0) {
      sum += parseInt(b[i])
    }

    result.unshift(parseInt(sum) % 2)
    carry = parseInt(sum) / 2
    i--
    j--
  }

  if (carry > 0) {
    result.unshift(1)
  }
  // console.log(result)
  return result.toString()
};

abbBinary('11', '1')