function decimalToOctal (num) {
  let oct = 0
  let c = 0
  while (num > 0) {
    const r = num % 8
    oct = oct + (r * Math.pow(10, c++))
    num = Math.floor(num / 8) // basically /= 8 without remainder if any
  }
  console.log('The decimal in octal is ' + oct)
}

decimalToOctal(2)
decimalToOctal(8)
decimalToOctal(65)
decimalToOctal(216)
decimalToOctal(512)