Digital Root

digital-root.js

function digitalRoot(number) {
  let lengthCheck = ''
  const originalNumber = number
  
  function add(a, b) {
    this.a = parseInt(a)
    this.b = parseInt(b)
    return this.a + this.b
  }
  
  lengthCheck =  originalNumber.toString().split('').reduce(add, 0)  
  return lengthCheck.toString().length > 1 ? digitalRoot(lengthCheck) : lengthCheck
}

console.clear()
console.log(digitalRoot(12345699999))