chroma-js/src/utils/contrast.js

JavaScript library for color conversions

const Color = require('../Color');
require('../ops/luminance');

module.exports = (a, b) => {
    // WCAG contrast ratio
    // see http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
    a = new Color(a);
    b = new Color(b);
    const l1 = a.luminance();
    const l2 = b.luminance();
    return l1 > l2 ? (l1 + 0.05) / (l2 + 0.05) : (l2 + 0.05) / (l1 + 0.05);
}