WCAG 2.0 contrast ratio implementation in JavaScript/ES6

contrast_checker.js

/**
 * Get the contrast ratio between two colors specified in RGB, as defined by WCAG 2.0.
 * Ex:
 *
 *     getContrastRatio([0, 0, 0], [255, 255, 255]);  // => 21 (white vs black, i.e. full contrast)
 *     getContrastRatio([0, 0, 0], [255, 0, 0]);      // => 5.252 (red vs black)
 *
 * References/resources:
 *   https://www.w3.org/TR/WCAG20/#contrast-ratiodef
 *   https://www.w3.org/TR/WCAG20/#relativeluminancedef
 *   https://colorable.jxnblk.com
 *   https://webaim.org/resources/contrastchecker/
 */
const getContrastRatio = (() => {
  const srgbFrom8Bit = val => val / 255;
  
  const getLuminanceComponent = srgb => srgb <= 0.03928 ? srgb / 12.92 : Math.pow((srgb + 0.055) / 1.055, 2.4);
  
  const getRelativeLuminance = (r, g, b) => 0.2126 * getLuminanceComponent(srgbFrom8Bit(r)) +
                                            0.7152 * getLuminanceComponent(srgbFrom8Bit(g)) +
                                            0.0722 * getLuminanceComponent(srgbFrom8Bit(b));
  
  const darkerOf = (c1, c2) => c1.reduce((a, e) => a + e) >= c2.reduce((a, e) => a + e) ? c2 : c1;
  const lighterOf = (c1, c2) => c1.reduce((a, e) => a + e) >= c2.reduce((a, e) => a + e) ? c1 : c2;
  
  return (c1, c2) => (getRelativeLuminance(...lighterOf(c1, c2)) + 0.05) /
                     (getRelativeLuminance(...darkerOf(c1, c2)) + 0.05);
})();