index.js

const rgb = require("rgb");
const wcag = require("wcag-contrast");
const isBlank = require("is-blank");
const isNamedCssColor = require("is-named-css-color");
const cssColorNames = require("css-color-names");

module.exports.ratio = ratio;
module.exports.score = score;
module.exports.isAccessible = isAccessible;
module.exports.isNotTransparent = isNotTransparent;

function ratio(colorOne, colorTwo, options) {
  colorOne = getRgbTriplet(colorOne, options);
  colorTwo = getRgbTriplet(colorTwo, options);

  return wcag.rgb(colorOne, colorTwo);
}

function score(colorOne, colorTwo, options) {
  var wcagScore = wcag.score(ratio(colorOne, colorTwo, options));

  if (isBlank(wcagScore)) {
    return "F";
  } else {
    return wcagScore;
  }
}

function isAccessible(colorOne, colorTwo, options) {
  return ratio(colorOne, colorTwo, options) > 4.5;
}

function getRgbTriplet(providedColor, options) {
  if (typeof providedColor !== "string") {
    throw new TypeError("get-contrast expects colors as strings");
  }

  let color = providedColor.toLowerCase()

  if (isNamedCssColor(color)) {
    color = cssColorNames[color];
  }

  color = isNotTransparent(color, options);
  return color
    .match(/\((.*)\)/)[1]
    .split(",")
    .slice(0, 3);
}

function isNotTransparent(color, options) {
  options = options || {};

  // Convert to RGB.
  color = rgb(color);
  // Check if the rgb returned color is rgba and if the 'a' value is 0.
  const cArray = color.match(/\((.*)\)/)[1].split(",");
  if (cArray.length == 4 && cArray[3] == "0" && !options.ignoreAlpha) {
    throw new TypeError("get-contrast cannot contrast transparent colors");
  } else {
    return color;
  }
}