Sorting strings in reverse order with backbone.js

Question

I'm trying to sort a Backbone.js collection in reverse order. There are previous replies on how to do this with integers, but none with strings.

var Chapter  = Backbone.Model; var chapters = new Backbone.Collection;  chapters.comparator = function(chapter) {   return chapter.get("title"); };  chapters.add(new Chapter({page: 9, title: "The End"})); chapters.add(new Chapter({page: 5, title: "The Middle"})); chapters.add(new Chapter({page: 1, title: "The Beginning"}));  alert(chapters.pluck('title')); 

The above code sorts the chapters from A -> Z, but how do I write a comparator that sorts it from Z -> A?

Answer 1

You could:

• grab the char code for each character in the string,
• subtract each value from 0xffff (the maximum return value of string.charCodeAt),
• use String.fromCharCode to turn that back into string of "negated" characters

and that will be your sorting key.

chapters.comparator = function(chapter) {     return String.fromCharCode.apply(String,         _.map(chapter.get("title").split(""), function (c) {             return 0xffff - c.charCodeAt();         })     ); } 

And voila:

> console.log(chapters.pluck('title')); ["The Middle", "The End", "The Beginning"] 

Note: if your comparison strings are long (as in 65 kb or more), you may run into trouble (see Matt's comment below). To avoid this, and speed up comparisons a bit, just use a shorter slice of your comparison string. (In the above example, you could go for chapter.get("title").slice(0, 100).split("") instead.) How long a slice you need will depend on your application.

Answer 2

There are two versions of the comparator function that you can use, either the sortBy version - which was shown in the example, which takes one parameter, or sort - which you can return a more standard sort function, which the documentation says:

"sortBy" comparator functions take a model and return a numeric or string value by which the model should be ordered relative to others. "sort" comparator functions take two models, and return -1 if the first model should come before the second, 0 if they are of the same rank and 1 if the first model should come after.

So in this case, we can write a different comparator function:

var Chapter  = Backbone.Model; var chapters = new Backbone.Collection;  chapters.comparator = function(chapterA, chapterB) {   if (chapterA.get('title') > chapterB.get('title')) return -1; // before   if (chapterB.get('title') > chapterA.get('title')) return 1; // after   return 0; // equal };  chapters.add(new Chapter({page: 9, title: "The End"})); chapters.add(new Chapter({page: 5, title: "The Middle"})); chapters.add(new Chapter({page: 1, title: "The Beginning"}));  alert(chapters.pluck('title')); 

So you should get as a response:

"The Middle", "The End", "The Beginning"