Possible solution for the sum interview question

sum.js

const sum(a, b) => {
  const maxLength = Math.max(a.length, b.length)
  const out = []
  for (let i = 0, carry = false; i < maxLength || carry; i++) {
    const numA = i < a.length ? parseInt(a[a.length - 1 - i], 10) : 0
    const numB = i < b.length ? parseInt(b[b.length - 1 - i], 10) : 0
    const sum = numA + numB + (carry ? 1 : 0)
    carry = sum >= 10
    out.unshift(sum % 10)
  }
  return out.join('')
}