Ligthen Darken Hex Colors

lightenDarkenHex.js

// helper functions are from https://github.com/rpearce/hex

const lightenDarken = (direction, color) => {
  const rgb = hexToRgba(color).slice(0, 3)
  
  // algo from https://stackoverflow.com/a/21038522/680394
  const rgbAdjusted = direction >= 0
    ? rgb.map(x => (1 - direction) * x + direction * 255)
    : rgb.map(x => (1 + direction) * x)
  
  return rgbaToHex(rgbAdjusted).slice(0, 7)
}

const hexToRgba = x => {
  const hex = parse(x)
  const head = hex.slice(0, 6)
  const tail = hex.slice(6)
  const n = parseInt(head, 16)
  return [(n >> 16) & 0xff, (n >> 8) & 0xff, (n >> 0) & 0xff, getAlpha(tail)]
}

const rgbaToHex = ([r = 0, g = 0, b = 0, a = 1] = []) => {
  const rgbN = (1 << 24) | (r << 16) | (g << 8) | b
  const hex = rgbN.toString(16).slice(1)
  const alpha = (a * 255)
    .toString(16)
    .padStart(2, '0')
    .slice(0, 2)
  return `#${hex}${alpha}`
}

const parse = (x = '') => {
  const hex = x.replace(/^#/, '')
  const len = hex.length
  return len === 3 || len === 4 ? repeat(hex) : hex
}

const repeat = x =>
  x.split('').map(y => y + y).join('')

const getAlpha = x =>
  x ? parseFloat((parseInt(x, 16) / 0xff).toFixed(2)) : 1

lightenDarken(0.9, '#DF3894') // "#fbebf4"

Alternative rgbaToHex function that may yield a slightly different result (and doesn't use alpha):

const rgbToHex = (rgb = []) =>
  '#' + rgb.map(x =>
    Math
      .min(255, Math.max(0, Math.round(x)))
      .toString(16)
      .padStart(2, '0')
  ).join('')

rgbToHex([223,56,148]) // "#df3894"