rgbaToHexConsole.js

// $0 is web inspector's reference to the current element
$0.style.backgroundColor
  // run a regex to get the values for r, g, b, and optionally a
  .match(/rgba?\((\d{1,3}), ?(\d{1,3}), ?(\d{1,3})(, ?([0-9.]{1,3}))?\)/)
  // filter out the regex results we don't care about
  .filter((e, i) => {
    switch (i) {
      case 1: // red
      case 2: // green
      case 3: // blue
      case 5: // alpha (group 4 is the optional capture group surrounding it)
        return true
        break
      default: // ignore everything else
        return false
    }
  })
  // Now we reduce the array of values to a HEX string
  .reduce(
    (acc, curr) =>
      // if alpha is undefined we want to just return the hex code
      // also, whole-number values need to be padded for a full HEX code
      curr !== undefined ? acc + Number(curr).toString(16).padEnd(2, "0") : acc,
    "#"
  )