relative-contrast-luminance.js

var chromatism = require("chromatism")

const blue = chromatism.convert("#0000ff");
const white = chromatism.convert("#ffffff");

function channelLuminance(channel) {
    const c = channel / 255;

    if (c <= 0.03928) {
      return c / 12.92;
    }

    return Math.pow((c + 0.055) / 1.055, 2.4);
}

// http://www.w3.org/WAI/GL/wiki/Relative_luminance
function relativeLuminance({ r, g, b }) {
  return 0.2126 * channelLuminance(r) + 0.7152 * channelLuminance(g) + 0.0722 * channelLuminance(b);
}

// http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html#key-terms
function contrast(color1, color2) {
  const L1 = relativeLuminance(color1);
  const L2 = relativeLuminance(color2);

  if (L1 < L2) {
    return (L2 + 0.05) / (L1 + 0.05);
  }

  return (L1 + 0.05) / (L2 + 0.05);
}

contrast(white.rgb, blue.rgb)