RGB, HSV, and HSL color conversion algorithms in JavaScript

color-conversion-algorithms.js

/**
 * Converts an RGB color value to HSL. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes r, g, and b are contained in the set [0, 255] and
 * returns h, s, and l in the set [0, 1].
 *
 * @param   Number  r       The red color value
 * @param   Number  g       The green color value
 * @param   Number  b       The blue color value
 * @return  Array           The HSL representation
 */
function rgbToHsl(r, g, b) {
  r /= 255, g /= 255, b /= 255;

  var max = Math.max(r, g, b), min = Math.min(r, g, b);
  var h, s, l = (max + min) / 2;

  if (max == min) {
    h = s = 0; // achromatic
  } else {
    var d = max - min;
    s = l > 0.5 ? d / (2 - max - min) : d / (max + min);

    switch (max) {
      case r: h = (g - b) / d + (g < b ? 6 : 0); break;
      case g: h = (b - r) / d + 2; break;
      case b: h = (r - g) / d + 4; break;
    }

    h /= 6;
  }

  return [ h, s, l ];
}

/**
 * Converts an HSL color value to RGB. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes h, s, and l are contained in the set [0, 1] and
 * returns r, g, and b in the set [0, 255].
 *
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  l       The lightness
 * @return  Array           The RGB representation
 */
function hslToRgb(h, s, l) {
  var r, g, b;

  if (s == 0) {
    r = g = b = l; // achromatic
  } else {
    function hue2rgb(p, q, t) {
      if (t < 0) t += 1;
      if (t > 1) t -= 1;
      if (t < 1/6) return p + (q - p) * 6 * t;
      if (t < 1/2) return q;
      if (t < 2/3) return p + (q - p) * (2/3 - t) * 6;
      return p;
    }

    var q = l < 0.5 ? l * (1 + s) : l + s - l * s;
    var p = 2 * l - q;

    r = hue2rgb(p, q, h + 1/3);
    g = hue2rgb(p, q, h);
    b = hue2rgb(p, q, h - 1/3);
  }

  return [ r * 255, g * 255, b * 255 ];
}

/**
 * Converts an RGB color value to HSV. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSV_color_space.
 * Assumes r, g, and b are contained in the set [0, 255] and
 * returns h, s, and v in the set [0, 1].
 *
 * @param   Number  r       The red color value
 * @param   Number  g       The green color value
 * @param   Number  b       The blue color value
 * @return  Array           The HSV representation
 */
function rgbToHsv(r, g, b) {
  r /= 255, g /= 255, b /= 255;

  var max = Math.max(r, g, b), min = Math.min(r, g, b);
  var h, s, v = max;

  var d = max - min;
  s = max == 0 ? 0 : d / max;

  if (max == min) {
    h = 0; // achromatic
  } else {
    switch (max) {
      case r: h = (g - b) / d + (g < b ? 6 : 0); break;
      case g: h = (b - r) / d + 2; break;
      case b: h = (r - g) / d + 4; break;
    }

    h /= 6;
  }

  return [ h, s, v ];
}

/**
 * Converts an HSV color value to RGB. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSV_color_space.
 * Assumes h, s, and v are contained in the set [0, 1] and
 * returns r, g, and b in the set [0, 255].
 *
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  v       The value
 * @return  Array           The RGB representation
 */
function hsvToRgb(h, s, v) {
  var r, g, b;

  var i = Math.floor(h * 6);
  var f = h * 6 - i;
  var p = v * (1 - s);
  var q = v * (1 - f * s);
  var t = v * (1 - (1 - f) * s);

  switch (i % 6) {
    case 0: r = v, g = t, b = p; break;
    case 1: r = q, g = v, b = p; break;
    case 2: r = p, g = v, b = t; break;
    case 3: r = p, g = q, b = v; break;
    case 4: r = t, g = p, b = v; break;
    case 5: r = v, g = p, b = q; break;
  }

  return [ r * 255, g * 255, b * 255 ];
}

It does work, but it is not good to do the comparison of floating point Number values as you do in your switch. If it works in your case, this is only a potentially unreliable special case. You could figure out the three cases of your switch before your /= expressions. With minimal modifications of your code, it would look like

const rgbToHsl = (r, g, b, a) => {
        let [r, g, b, a] = rgba;
        let max = Math.max(r, g, b), min = Math.min(r, g, b);
        const componentCase = max == r ? 0 : (max == g ? 1 : 2); // is max r g or b?
        r /= 255, g /= 255, b /= 255, min /= 255, max /= 255;
        let h, s, l = (max + min) / 2;
        if (max == min) {
            h = s = 0; // achromatic
        } else {
            let d = max - min;
            s = l > 0.5 ? d / (2 - max - min) : d / (max + min);
            switch (componentCase) {
                case 0: h = (g - b) / d + (g < b ? 6 : 0); break;
                case 1: h = (b - r) / d + 2; break;
                case 2: h = (r - g) / d + 4; break;
            }
            h /= 6;
        }
        return [h, s, l, a];
};

Also, note that you should use const or let, but not var, to avoid the known risks related to the obsolete var.
Then, it is not nice that you accept separate r, g, and b on input but the function returns an array. Why RGB should be different from HSL? Note the line let [r, g, b, a] = rgba.

Finally, it is good to have the alpha channel as well. This work is done for the use in CSS, where the alpha channel is generally used everywhere.

Here's a version of hslToRgb for Clojure (and ClojureScript and whatever Clojure implementation):

(defn hsl->rgb [h s l]
  (let [hue->rgb (fn hue->rgb
                   [p q t]
                   (let [t (if (< t 0) (inc t) t)
                         t (if (> t 1) (dec t) t)]
                     (cond
                       (< t (/ 1 6)) (+ p (* (* (- q p) 6) t))
                       (< t (/ 1 2)) q
                       (< t (/ 2 3)) (+ p (* (* (- q p) (- (/ 2 3) t)) 6))
                       :else p)))
        [r g b] (if (= s 0)
                  [l l l]
                  (let [q (if (< l 0.5) (* l (+ 1 s)) (- (+ l s) (* l s)))
                        p (- (* 2 l) q)]
                    [(hue->rgb p q (+ h (/ 1 3))) (hue->rgb p q h) (hue->rgb p q (- h (/ 1 3)))]))]
    [(* r 255) (* g 255) (* b 255)]))

(comment
  (hsl->rgb (/ 120 256) 1.0 0.5)
  ;; => [0 255 207.18750000000003]
  :rcf)

It's a straight port. There may be dragons such as boxed math and things? I haven't considered.

Thank you, Peter!

Here's a version of hslToRgb for Clojure (and ClojureScript and whatever Clojure implementation):