determine luminance by hex color

what-color-to-write.js

// HEX to RGB function
const hexToRgb = hex =>
  hex.replace(/^#?([a-f\d])([a-f\d])([a-f\d])$/i, (m, r, g, b) => `#${r + r + g + g + b + b}`)
    .substring(1)
    .match(/.{2}/g)
    .map(x => parseInt(x, 16))

// Determine relation of luminance in color
const luminance = (r, g, b) => {
  const a = [r, g, b].map((v) => {
    v /= 255
    return v <= 0.03928
      ? v / 12.92
      : ((v + 0.055) / 1.055) ** 2
  })

  return (a[0] * 0.2126 + a[1] * 0.7152 + a[2] * 0.0722)
}

// Using
const backgroundColor = '#0366d6'
const rgb = hexToRgb(backgroundColor) // make rgb from hex, got array of r, g, b
// push the array by spread operator to function
// if spread doesn't work or not awailable, use luminance(rgb[0], rgb[1], rgb[2])
const lum = luminance(...rgb)
const textColor = (lum < 0.228) ? '#FFFFFF' : '#000000'