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Alphanum: Javascript Natural Sorting Algorithm

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Today I had a need for a natural sorting function in javascript. I've seen these functions before in other languages, so I thought to myself, no problem, I'll find one in a couple of minutes!. How wrong I was!

First off, a little explanation. Natural sorting, or Lexicographical order, is sorting a list of strings which contain letters and numbers, in such a way that makes sense to humans rather than computers. Given a list of filenames, a computer would sort them this way:

z1.doc, z10.doc, z17.doc, z2.doc, z23.doc, z3.doc
While a human would sort it like so:

z1.doc, z2.doc, z3.doc, z10.doc, z17.doc, z23.doc
Obviously, the second list makes more sense, no? Anyway, I needed an algorithm that would sort any list of strings in that fashion.

Right off the bat, I found two promising candidates, but both failed quite spectacularly on my test array (which included bunches of numbers up to 1000). More searching only seemed to find blog posts which merely linked to these faulty examples.

So, giving up that angle, I decided the best way to get a natural sorting algorithm in javascript was to examine one from another language (preferably one which worked) and port it to javascript.

I ended up coming across David Koelle's Alphanum algorithm which worked like a charm. I took the Perl implementation and ported it to Javascript resulting in the function below:

function alphanum(a, b) {
  function chunkify(t) {
    var tz = [], x = 0, y = -1, n = 0, i, j;

    while (i = (j = t.charAt(x++)).charCodeAt(0)) {
      var m = (i == 46 || (i >=48 && i <= 57));
      if (m !== n) {
        tz[++y] = "";
        n = m;
      tz[y] += j;
    return tz;

  var aa = chunkify(a);
  var bb = chunkify(b);

  for (x = 0; aa[x] && bb[x]; x++) {
    if (aa[x] !== bb[x]) {
      var c = Number(aa[x]), d = Number(bb[x]);
      if (c == aa[x] && d == bb[x]) {
        return c - d;
      } else return (aa[x] > bb[x]) ? 1 : -1;
  return aa.length - bb.length;
Note that this code is case sensitive, so "A" will sort before "a". You can turn it into a case insensitive sort by lowercasing the incoming a and b test strings. Just change the chunkify lines to:

  var aa = chunkify(a.toLowerCase());
  var bb = chunkify(b.toLowerCase());

Thanks to David K. for the algorithm! This saved me tons of time. I owe you one. smile

An IE Issue Appears
After first posting this script, I came across an issue in IE which caused the script to fail. Don't worry! The lines of code above are the modified - and working - alphanum function. The problem occurred in the script as originally posted; specifically with the two lines:

  a = chunkify(a);
  b = chunkify(b);
Note the variable names, where the assignment tries to place the result back into the original variable. Actually, all that's required for the bug to manifest is to assign any array to the incoming a and b variables. For some strange reason this would fail in IE6 and IE7 while sorting arrays with larger than 23 or 24 elements. In these cases, at seemingly random times during the sort, "undefined" would be passed back to a, causing the next reference to it to trigger an exception.

The following is example code (from a testcase prepared by TarquinWJ) which triggers the bug in IE. This sort will fail in IE6 and IE7.

function customsort(a, b) {
  a = ['a']; // assign an array to a
  b = ['b']; // assign an array to b

  // IE will bail out when either a or b is undefined
  return a.length - b.length;
var arr = [
To solve this, all it took was to assign the result to new variables (aa and bb) and change all further references to a and b to the new set. Definitely a JS bug in IE, and as it involves the creation and destruction of many arrays within a short time, it is likely some kind of memory issue.

Thanks very much for TarquinWJ for helping isolate this bug.

A Faster Prototype Version
For those who value speed over flexibility, I also made this function into an Array method. Because it breaks up all the array strings first, then does the actual sorting, then joins all the sub-arrays back into strings, it is a great deal faster than the version above. That version breaks up two strings for each sort iteration rather than only once for each array element.

It's harder to modify this method to sort multi-dimensional arrays, or arrays of objects with string properties, but the speed gain is hard to ignore. As an added bonus, you can specify case sensitivity on a per-call basis with the caseInsensitive argument (true|false). If you're just going to sort plain arrays of strings, use this version.

Array.prototype.alphanumSort = function(caseInsensitive) {
  for (var z = 0, t; t = this[z]; z++) {
    this[z] = [];
    var x = 0, y = -1, n = 0, i, j;

    while (i = (j = t.charAt(x++)).charCodeAt(0)) {
      var m = (i == 46 || (i >=48 && i <= 57));
      if (m !== n) {
        this[z][++y] = "";
        n = m;
      this[z][y] += j;

  this.sort(function(a, b) {
    for (var x = 0, aa, bb; (aa = a[x]) && (bb = b[x]); x++) {
      if (caseInsensitive) {
        aa = aa.toLowerCase();
        bb = bb.toLowerCase();
      if (aa !== bb) {
        var c = Number(aa), d = Number(bb);
        if (c == aa && d == bb) {
          return c - d;
        } else return (aa > bb) ? 1 : -1;
    return a.length - b.length;

  for (var z = 0; z < this.length; z++)
    this[z] = this[z].join("");

PlanetWerks 2 - Now Available!How to copy arrays and objects in Javascript


RamūnasRamunas Wednesday, January 16, 2008 10:03:36 PM

Nice smile
Finding that bug in IE sounds like a serious PITA though sad

Brian HuismanGreyWyvern Wednesday, January 16, 2008 10:16:01 PM

Hehe. Just when you think you can have your cake and eat it too, along comes IE, everytime!

wnu Monday, September 1, 2008 9:02:31 PM

I whipped up another version of a natural sort function in Javascript that works well and is less code, considerably faster (with more complex alpha-numeric values) and doesn't use the charIndex comparisons:

Using the same Dave Koelle "chunking" idea that seems to more mimic "String Tokenization" or "Lexical Analysis" and then sorting off the tokens using the default browser comparison operations.

Still - love the port of Dave's algorithm and was great to dig through the logic! Well done!

Emily Hoangemilyhoang Friday, January 28, 2011 7:07:25 PM

Hello, I want to use your js file in one of my product. Is there any copyright restrictions that I need to be aware of? As long as I leave everything as is I should be free to use it?

mrDoktar Monday, April 4, 2011 10:21:53 AM


I wrote an alternative to your algorithm:

Array.prototype.humanSort = function() {
  return this.sort(function(a, b) {
    aa = a.split(/(\d+)/);
    bb = b.split(/(\d+)/);

    for(var x = 0; x < Math.max(aa.length, bb.length); x++) {
      if(aa[x] != bb[x]) {
        var cmp1 = (isNaN(parseInt(aa[x],10)))? aa[x] : parseInt(aa[x],10);
        var cmp2 = (isNaN(parseInt(bb[x],10)))? bb[x] : parseInt(bb[x],10);
        if(cmp1 == undefined || cmp2 == undefined)
          return aa.length - bb.length;
          return (cmp1 < cmp2) ? -1 : 1;
    return 0;

Variety is the spice of life smile

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