Unique Array Filter in Dojo

I was just doing some work that needed some array parsing in JavaScript. I was surprised that I couldn’t find anything that seemed relevant in the Dojo source that would allow me to do this. It’s one of those rare occasions where I assumed Dojo would have it and it didn’t, rather than the usual spend hours coding something to find out I could have just done dojo.doAwesomeStuff() 2 hours earlier.

So I set about trying to find a solution. The problem? I have an array that contains items, I want them to be unique. I want to filter out all the ones that are the same. I want an array_unique like they have in PHP.

I found an interesting chunk of code over here on a blog post that’s about a different problem being solved with Dojo.

Amongst all the other stuff was this snippet:

cleanFilterValues:function(values) {
var unique = {};
//get rid of duplicates
return dojo.filter(values, function(value) {
if (!unique[value]) {
unique[value] = true;
return true;
}
return false;
}).sort();
},

I rather liked the look of it. It was super simple. Didn’t need you to loop through the array twice (once to get tha array value and then once over the whole thing again to see if the value you had matched one already in the array). Instead You just sort them all into order and go through them one at a time and don’t return any that are the same as the one before it.

Now of course this won’t work everywhere.

It won’t work where you can’t sort the array for example.

But for what I needed it was perfect. I figured I’d share it.

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Published:: 08.16.08 / 1am

Category:: Front End Code

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maulin 08.16.08 / 1am

Actually, I don’t think you need to worry about the sortability of the array. This will work for any array whose items could be keys to an associative array (strings or numeric). Or am I missing something?

JP 08.16.08 / 2am

True. On first pass I thought it was doing the check differently, I thought it was actually looking to see if a value was previously set. As you say, all it really does it looks to see if there is any value with that value using the array['key'] lookup.

It’s a great piece of code by the way, many thanks for sharing it.

bitranch 08.16.08 / 8am

I’ve used this for a while (which is basically the same thing). I borrowed the basic pattern from an old Python unique recipe in which a dict acts as the test object.

unique = function(/* Array */ arr){
  var test = {};
  return dojo.filter(arr, function(val){
      return test[val] ? false : (test[val] = true);
    });
};

bitranch 08.16.08 / 11am

You’d hope I’d have something better to do on a Saturday morning, but…

Here’s a non-dojo-specific and faster implementation. This is a better solution when dealing with large arrays. (For short arrays, it really doesn’t matter that much.)

unique2 = function(/* Array */ arr){ var test = {}; var result = []; for (var i=0,len=arr.length; i < len; i++){ if (!test[arr[i]]){ // value not seen yet? test[arr[i]] = true; result.push(arr[i]); } } return result; };

Note that none of these hash-based unique functions work for arrays of objects/hashes unless each object’s toString() function produces a unique value for the particular object (i.e. something based on content rather than “[object Object]” or “[object HTMLDivElement]“). So it can’t be used as-is to create a unique NodeList, for example.

Jon Sykes

Unique Array Filter in Dojo

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