I wrote this article months ago when I was first experimenting with Sass 3.3 alpha features. I came up with a pretty wild solution to generate a random number in Sass. However it looks like Sass 3.3 will implement a random function so we won’t need all this stuff. I still publish it for fun. :)

Everything started when I was spying on Sass 3.3 source code on GitHub for my article about the future of Sass at David Walsh' Blog. I was sniffing the incoming functions when all of the sudden I came by a unique-id() function.

According to the issue which started this idea, the unique-id() function should return a unique random alphanumeric identifier that could be used for whatever you like. As far as I understood the example provided by Chris Eppstein, it could be used to dynamically generate and extend a placeholder from within a mixin. Kind of complicated stuff, really.

Anyway, I saw this unique id thingie as an opportunity to have a random number with Sass. Why? I don’t know. I leave this question to you. Maybe some day I’ll find a usecase for a random number in CSS.

Note: the code in this article has not been tested at all since it requires some Sass 3.3 functions that are not implemented yet. This is more like a proof of concept.

About unique-id()

To understand what this is all about, you need to know what the unique-id() is and what it returns. First of all, there are two different functions for this in Sass source code, both from 2 months ago: one in tree f3be0f40b7 (using base36) and one in branch unique_id (using base16). I only worked on the latter since it’s most likely this is the one that will be implemented.

I’m not a Ruby pro, but with the help of a kind folk on Twitter, I could make it work on CodePad. Here is what a couple of run of the function looks like:


So the function returns a 19-characters long alphanumeric string. As you may have noticed, the returned string always starts with a u. This is actually hard-coded inside the function core to make sure the string always start with a letter in order to be able to be used as a class / placeholder / id, whatever.

To put it very simple, the function randoms a 19-digits number, convert it to base 16 (or base 36 in the other implementation), then append it a u. So when we use unique-id(), we end up with something like this: u8547f4133644af4c.

Random, the dirty way

My first attempt to get a random number from this string was to remove all alpha characters from it, then keep only the number of digits we want (or we still have). To do this, I used the incoming string manipulation functions (str-length(), str-slice(), str-insert()):

@function rand($digits: 16) {
  /* Array of characters to remove */
  $letters: a b c d e f u;
  $result: unquote('');
  $string: unique-id();

  /* For each character in the given string */
  @for $i from 1 through str-length($string) {
    /* Isolate character */
    $character: str-slice($string, $i, $i + 1);
    /* If not a letter */
    @if index($character, $letters) == false {
      /* Append it to $value */
      $value: str-insert($result, $character, str-length($result) + 1);

  /* Deal with the number of digits asked */
  @if $digits !== 0 and $digits < length($result) {
    $result: str-slice($result, 1, $digits);

  /* Return the result */
  @return $result;

I think the code is pretty much self-explanatory. I check each character individually: if it’s not a letter, I append it to the $result variable. When I’m done, if the length of $result is still greater than the number of digits we asked for ($digits) we truncate it.

And there we have a random number between 1 and 9999999999999999 (in case the 16 characters are 9).

$number: rand(); /* Random between 1 and 9999999999999999 */
$number: rand(1); /* Random between 1 and 9 */
$number: rand(4); /* Random between 1 and 9999 */
$number: rand(0); /* Random between 1 and 9999999999999999 */
$number: rand(-1); /* Random between 1 and 9999999999999999 */

Random, the clean way

Okay, let’s say it: the first version I came with is really dirty. That’s why I reworked a new version from scratch with the help of my brother. We even tweaked it in order to make it future-proof for both implementations of the unique-id() function. How cool is that?

To put it simple, instead of stripping alpha characters, we take the alphanumeric string and convert it back into an integer. Then, we get a fully random integer we simply have to manipulate around min and max values.

@function rand($min: 0, $max: 100) {
  $str: str-slice(unique-id(), 2);
  $res: toInt($str, 16);
  @return ($res % ($max - $min)) + $min;

The first line in the function core is the unique-id() function call. We immediately pass it into the str-slice() function to remove the very first character which is always a u.

Note: According to my tests, the min value used in both implementations of unique-id() is such that the second character of the returned string is always the same (8 in base 16, 1 in base 36). Thus we may need to strip it too, like this str-slice(unique-id(), 3).

The second line calls a toInt() function, passing it both the string ($str) and the base we want to convert the string from (not to). This is why I say we’re ready for both implementations: we only have to change this 16 to 36 and everything should work like a charm.

Before going to the last line, let’s have a look at the toInt function:

@function toInt($str, $base: 10) {
  $res: 0;
  $chars: charsFromBase($base);
  @if $chars !== false {
    $str: if($base < 64, to-lower-case($str), $str);
    @for $i from 1 through str-length($str) {
      $char: str-slice($str, $i, $i + 1);
      $charVal: index($char, $chars) - 1;
      $res: $res + pow(length($base), str-length($str) - $i) * $charVal;
    @return $res;
  @return false;

$res will store the result we will return once we’re done. $chars contains the array of characters used by base $base; we’ll see the charsFromBase() function right after. Then, if the base is supported we loop through each characters of the string.

For every character, we isolate it ($char) and convert it to its numeric equivalent ($charVal) thanks to the $chars array. Then, we multiply this number to the base raised to the reversed index in the string. That may sound a little complicated, let me rephrase it: in base 10, 426 equals 4*10^2 + 2*10^1 + 6*10^0. That’s pretty much what we do here, except instead of 10 we use the base, and instead of 2, 1 and 0, we use the length of string minus the index of the current character.

The pow() function used to raise a value to an exponent is part of Compass Math helpers. In case you don’t want to use Compass or simply can’t use Compass, here is the pow() function in pure Sass:

@function pow($val, $pow) {
  $res: 1;
  @while ($pow > 0) {
    $res: $res * $val;
    $pow: $pow - 1;
  @return $res;

And of course, we add this to the result ($res). Once we’re done with the string, we return the result to the rand() function. Then, we simply return ($res % ($max - $min)) + $min to the user resulting in a random number between min and max values.

Regarding the charsFromBase() function, here is what it looks like:

@function charsFromBase($base: 10) {
  /* Binary */
  @if $base == 2 {
    @return 0 1;
  /* Octal */
  @if $base == 8 {
    @return 0 1 2 3 4 5 6 7;
  /* Decimal */
  @if $base == 10 {
    @return 0 1 2 3 4 5 6 7 8 9;
  /* Hexadecimal */
  @if $base == 16 {
    @return 0 1 2 3 4 5 6 7 8 9 a b c d e f;
  /* Base 36 */
  @if $base == 36 {
    @return 0 1 2 3 4 5 6 7 8 9 a b c d e f g h i j k l m n o p q r s t u v w x
      y z;
  /* Base 64 */
  @if $base == 64 {
    @return A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h
      i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 + /;
  @return false;

I only added most common standard bases (binary, octal, decimal, hexadecimal, 36, 64) but I guess we could probably add a couple of others. Actually this is already too much since we know the unique-id() function will return a base16 or base36 encoded string (depending on the implementation they’ll keep).

Final words

That’s pretty much it. As I said at the beginning of the article, I couldn’t try this code since neither the unique-id() nor the string manipulation functions are currently implemented in the Sass 3.3 Alpha version. So this is pretty much blind coding here!

If you think of anything that could improve this Sass random function, please be sure to tell. Meanwhile you can play with the code directly on this pen.