calculateContrastRatio.js

const calculateRelativeLuminance = ({ r, g, b }) => {
  const [R, G, B] = [r, g, b]
    .map((bit) => bit / 255)
    .map(
      (sRGB) =>
        sRGB <= 0.03928 ? sRGB / 12.92 : Math.pow((sRGB + 0.055) / 1.055, 2.4),
    )
  return 0.2126 * R + 0.7152 * G + 0.0722 * B
}

const calculateContrastRatio = (a, b) => {
  const [L1, L2] = [a, b].map(calculateRelativeLuminance).sort((a, b) => b - a)
  return (L1 + 0.05) / (L2 + 0.05)
}

calculateContrastRatio({ r: 34, g: 34, b: 34 }, { r: 255, g: 255, b: 255 }) // 15.909984431773273