Find out if a color is light or dark.

light-or-dark.js

function lightOrDark(color) {
  let r, g, b, hsp;
  color = parseInt(
    '0x' + color.slice(1, color.length - 2).replace( color.length < 5 && /./g, '$&$&')
  );
  r = color >> 16;
  g = color >> 8 & 255;
  b = color & 255;
  hsp = Math.sqrt(0.299 * (r * r) + 0.587 * (g * g) + 0.114 * (b * b));
  return hsp > 127.5 ? 'light' : 'dark';
}