Iterative Implementation

add-number-array.js

/**
 * 654 + 59 = 713
 * addArrayNumbers([6, 5, 4], [5, 9]) // [7, 1, 3];
 *
 * @param {Array} first
 * @param {Array} second
 * @return {Array}
 */
function addArrayNumbers(first, second) {
  var result = [],
      carry = 0,
      maxLength = Math.max(first.length, second.length),
      a, b, sum, i;
  
  
  for (i = 1; i <= maxLength; i++) {
    a = first[first.length - i] || 0;
    b = second[second.length - i] || 0;
    sum = a + b + carry;
    
    if (sum >= 10) {
      carry = 1;
    }
    else {
      carry = 0
    }
    
    result.unshift(sum % 10);
  }
  
  if (carry > 0) {
    result.unshift(1);
  }
  

  return result;
}