function RGBToHex (r, g, b) {
  if (
    typeof r !== 'number' ||
    typeof g !== 'number' ||
    typeof b !== 'number'
  ) {
    throw new TypeError('argument is not a Number')
  }

  const toHex = n => (n || '0').toString(16).padStart(2, '0')

  return `#${toHex(r)}${toHex(g)}${toHex(b)}`
}

console.log(RGBToHex(255, 255, 255) === '#ffffff')
console.log(RGBToHex(255, 99, 71) === '#ff6347')