function hexToInt (hexNum) {
  const numArr = hexNum.split('') // converts number to array
  numArr.map((item, index) => {
    if (!(item > 0)) {
      switch (item) {
        case 'A': return (numArr[index] = 10)
        case 'B': return (numArr[index] = 11)
        case 'C': return (numArr[index] = 12)
        case 'D': return (numArr[index] = 13)
        case 'E': return (numArr[index] = 14)
        case 'F': return (numArr[index] = 15)
      }
    } else numArr[index] = parseInt(item)
  })
  return numArr // returns an array only with integer numbers
}

function hexToDecimal (hexNum) {
  const intItemsArr = hexToInt(hexNum)
  return intItemsArr.reduce((accumulator, current, index) => {
    return accumulator + (current * Math.pow(16, (intItemsArr.length - (1 + index))))
  }, 0)
}

// test cases
console.log(hexToDecimal('5DE9A')) // 384666
console.log(hexToDecimal('3D')) // 61