Expand a 3-character hex string (beginning with #) into a 6 character one

expandRGB.js

export function expandRGB(string) {
  if(!(/#[a-f0-9]{3}$/i).test(string)) return string;

  const rgb = string.split('');
  
  return `#${rgb[1]}${rgb[1]}${rgb[2]}${rgb[2]}${rgb[3]}${rgb[3]}`;
}