Return either white or black to achieve greatest constrast.

colorConstrast.js

function whiteOrBlack(color) {
  const colorArr = color.length === 4 ? color.match(/[\da-fA-F]{1}/g) : color.match(/[\da-fA-F]{2}/g);
  return colorArr
          .map(s => s.length > 1 ? parseInt(s, 16) : parseInt(s, 16) * 16)
          .reduce((a, c) => a + c) > 381 ? '#000' : '#FFF';
}