hexToRGB.js

function hexToRGB (hex) {
  
  // remove whitespace from left and rihgt ends
  hex = hex.trim()
  if (hex.startsWith('#')) {
    hex = hex.replace('#', '')
  }
  
  // convert short hand notation (f0f) to the regular notation (ff00ff)
  if (hex.length == 3) {
    let temp = ''
    for (let i = 0; i < hex.length; i++)
      temp += hex[i] + hex[i]
    hex = temp
  }

  // check for additional white space between characters (f    0    f)
  if (hex.length != 6 ) {
    throw new Error(`hexToRGB was called with incorrect hex code. 
      Valid hex code is either three to four characters long with following characters '0123456789ABCDEFabcdef' `)
  }

  // check for valid characters from 0-F
  let validHexChars = '0123456789ABCDEFabcdef' 
  for (let i = 0; i < hex.length; i++) {
      if ( !validHexChars.includes(hex[i]) ) {
        throw new Error(`hexToRGB was called with incorrect hex code. 
          Valid hex code is either three to four characters long with following characters '0123456789ABCDEFabcdef' `)        
      }
  }
  
  let RGB = []
  let r = parseInt(hex.substring(0, 2), 16)
  RGB.push(r)
  let g = parseInt(hex.substring(2, 4), 16)
  RGB.push(g)
  let b = parseInt(hex.substring(4, 6), 16)
  RGB.push(b)

  return RGB
}

//console.log( hexToRGB('ff00ff') ) // [255, 0, 255]
//console.log( hexToRGB('#ff00ff') ) // [255, 0, 255]
//console.log( hexToRGB('#f0f') ) // [255, 0, 255]
//console.log( hexToRGB('ff00fff') ) // hextToRGB was called with incorrect hex code. ...
//console.log( hexToRGB('ff00ffg') ) // hextToRGB was called with incorrect hex code. ...