gray-percentage/index.js

Convenience function to get a gray value by percentage e.g. gray(60) -> hsla(0,0%,100%,0.4)

function isNumeric(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}


module.exports = function(lightness, hue, darkBackground) {
  if (typeof hue === "undefined") {
    hue = 0;
  }
  if (typeof darkBackground === "undefined") {
    darkBackground = false;
  }

  // Convert named hues into numeric lightness value.
  if (hue === "cool") {
    hue = 237;
  }
  else if (hue === "slate") {
    hue = 122;
  }
  else if (hue === "warm") {
    hue = 69;
  }

  if (!isNumeric(hue)) {
    throw new Error("Hue is not a number");
  }

  if (!isNumeric(lightness)) {
    throw new Error("Lightness is not a number");
  }

  if (lightness > 100) {
    lightness = 100;
  }
  if (lightness < 0) {
    lightness = 0;
  }

  var saturation = 0;
  if (hue !== 0) {
    var a = 19.92978;
    var b = -0.3651759;
    var c = 0.001737214;
    saturation = a + b * lightness + c * Math.pow(lightness, 2);
  }

  var opacity = 0
  if (darkBackground) {
    opacity = lightness / 100
    lightness = '100%,'
  } else {
    opacity = (100 - lightness) / 100
    lightness = '0%,'
  }

  return "hsla(" + hue + "," + saturation + "%," + lightness + opacity + ")";
};