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Last Updated: February 25, 2016
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· weslleyaraujo

Sorting a Backbone.js collection

If you know some about Backbone.js you also probably know that it offers you something pretty cool which are the collections. Those are kind of a wrapper for dealing with models in a easy way giving you awesome features, and one of those features is that you can sort those models to proper work with. I'm gonna talk about the comparator method and what it can help you.

Let's start creating a piece of code, a very simple Backbone collection:

var CarsCollection = Backbone.Collection.extend();

var cars = new CarsCollection();

Well done,

now we have our collection ready to add some data into it:

var CarsCollection = Backbone.Collection.extend();

var cars = new CarsCollection();

cars.add({ name: 'Gol', year: 1998 });
cars.add({ name: 'Camaro', year: 2012 });
cars.add({ name: 'Palio', year: 2014 });
cars.add({ name: 'Ferrari', year: 2011 });
cars.add({ name: 'Fusca', year: 1998 });

Okay now our collection has some data and it can do some tricks, let's starting by retrieving only the year attribute from it. We are going to use the pluck method, that basically plucks an attribute from the collection and returns in a array.

So what we get is this:

cars.pluck('year'); // [1998, 2012, 2014, 2011, 1998]
cars.pluck('name'); // ["Gol", "Camaro", "Palio", "Ferrari", "Fusca"]

Notice that by the log of the name and the year we can see that this collection is sorted in the same way we added those items above, lets now write your comparator to make it work in a different way:

var CarsCollection = Backbone.Collection.extend({
  comparator: function (a) {
    return a.get('year');
  }

});

var cars = new CarsCollection();

cars.add({ name: 'Gol', year: 1998 });
cars.add({ name: 'Camaro', year: 2012 });
cars.add({ name: 'Palio', year: 2014 });
cars.add({ name: 'Ferrari', year: 2011 });
cars.add({ name: 'Fusca', year: 1998 });

and now using pluck let's see what happened:

cars.pluck('year'); // [1998, 1998, 2011, 2012, 2014]
cars.pluck('name'); // ["Gol", "Fusca", "Ferrari", "Camaro", "Palio"]

Awesome! In very a simple line of code we made this collection be sorted by the year attribute... But how does that work?

The comparator is called everytime you add an in item to the collection or by calling sort(). It will automatically receives two parameters, and those are the models which you can compare with. In our case we just had to return the chosen attribute and Backbone does the trick.

But lets say we need some more logic here, maybe if we also need to sort those cars alphabetically by the name, how do we do that?

You can see that comparator works in a very similar way with the Array.prototype.sort, that basically you have to return a number positive, negative, or 0 for a equal sort . You can read more about this awesome method here.

So to do that our comparator should be written like this:

var CarsCollection = Backbone.Collection.extend({
  comparator: function (a, b) {
      var year = a.get('year') - b.get('year');
      if (year === 0) {
          return a.get('name') < b.get('name') ? -1 : 1;
        }

      return year;
  }
});

var cars = new CarsCollection();

cars.add({ name: 'Gol', year: 1998 });
cars.add({ name: 'Camaro', year: 2012 });
cars.add({ name: 'Palio', year: 2014 });
cars.add({ name: 'Ferrari', year: 2011 });
cars.add({ name: 'Fusca', year: 1998 });

And now if we pluck our data:

cars.pluck('year'); // [1998, 1998, 2011, 2012, 2014]
cars.pluck('name'); // ["Fusca", "Gol", "Ferrari", "Camaro", "Palio"]

We did it! Now our data are sorted by year and by the name of the car!

First we subtract the a year by the b year, and if the result its not zero (which would be zero if those are the same) we just return this subtraction result, in the other hand we need to get the name of those models and compare them, returning which one should be on first place than the other, and voilà we did exactly what we wanted.

Conclusion

Backbone has lots and lots of tricks, and the best part of it is that those tricks are nothing like magic, if you understand javascript you could quickly read this code and in minutes you would know what is going on.I believe that some kind of data sorting should be on the server side, but if you ever need to do that in the client side it's also easy to do.

If you wanna learn more of those Backbone tricks I highly recommend the Advanced Backbone Patterns and Techniques from Tutsplus.

You can follow my posts on my Blog