Color comparison function for RGB (isLighterThan, isDarkerThan)

color-comparison.js

/** Extracted from https://gist.github.com/alexandrudima/bda598fbed179a581986b634cc94ab8d */

/**
 * Returns the number in the set [0, 1]. 
 *   - O => Darkest Black. 
 *   - 1 => Lightest white.
 * @see http://www.w3.org/TR/WCAG20/#relativeluminancedef
 *
 * @param {Object} color
 * @param {Number} color.r
 * @param {Number} color.g
 * @param {Number} color.b
 * @return {Number}
 */
function getRelativeLuminance(color) {
  const luminance = 0.2126 * _relativeLuminanceForComponent(color.r) 
                  + 0.7152 * _relativeLuminanceForComponent(color.g) 
                  + 0.0722 * _relativeLuminanceForComponent(color.b);
  const decimal = Math.pow(10, 4);
  return Math.round(luminance * decimal) / decimal;
}

function _relativeLuminanceForComponent(n) {
  const c = n / 255;
  return (c <= 0.03928) ? c / 12.92 : Math.pow(((c + 0.055) / 1.055), 2.4);
}

function isLighterThan(a, b) {
  return getRelativeLuminance(a) > getRelativeLuminance(b);
}

function isDarkerThan(a, b) {
  return getRelativeLuminance(a) < getRelativeLuminance(b);
}

// Usage
isLighterThan({ r: 255, g: 250, b: 200 }, {r: 255, g: 250, b: 0 }); // true