contrast-ratio.js

function hexToRgb(hex) {
  var shorthandRegex = /^#?([a-f\d])([a-f\d])([a-f\d])$/i;
  hex = hex.replace(shorthandRegex, function (m, r, g, b) {
    return r + r + g + g + b + b;
  });

  var result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
  return result ? {
    r: parseInt(result[1], 16),
    g: parseInt(result[2], 16),
    b: parseInt(result[3], 16)
  } : null;
}

function luminance(r, g, b) {
  var a = [r, g, b].map(function (v) {
    v /= 255;
    return v <= 0.03928
      ? v / 12.92
      : Math.pow((v + 0.055) / 1.055, 2.4);
  });
  return a[0] * 0.2126 + a[1] * 0.7152 + a[2] * 0.0722;
}

const color1rgb = hexToRgb('#ffffff');
const color2rgb = hexToRgb('#B30000');

const color1luminance = luminance(color1rgb.r, color1rgb.g, color1rgb.b);
const color2luminance = luminance(color2rgb.r, color2rgb.g, color2rgb.b);

const ratio = color1luminance > color2luminance
  ? ((color2luminance + 0.05) / (color1luminance + 0.05))
  : ((color1luminance + 0.05) / (color2luminance + 0.05));

console.log(ratio)
console.log((Math.floor(100 / ratio)) / 100 + ' : 1')