Count and sort by number of occurrences

Question

I have an array like this:

var items = [
  {name: 'popcorn', category: 'snacks'},
  {name: 'nachos', category: 'snacks'},
  ...
  {name: 'coke', category: 'drinks'}
];

I want to get a list of categories ordered by the number of items on it (most items first):

['snacks', 'drinks']

Here is how I did it:

var categories = _.chain(items)
  .countBy(function (i) { return i.category })
  .pairs()
  .sortBy(function (c) { return -c[1] })
  .map(function (c) { return c[0] })
  .value();

So the result of countBy is an object, and I have to use pairs to convert it into an array of arrays in order to sort it.

I wonder if there is a more straightforward way to do it? Built-in function?

Answer 1

I wonder if there is a built-in function?

Nope, your approach is fine.

more straightforward way to do it?

You could shorten it a bit by passing only property names, not functions:

var categories = _.chain(items)
  .countBy("category")
  .pairs()
  .sortBy(1).reverse()
  .pluck(0)
  .value();

Answer 2

I think this is pretty good. You could cram it into a one-step reduce but that would wind up being more complicated than what your code looks like now. If you're going to be reusing this a bunch, you could use compose and partial to reduce the overall amount of code you're writing. Also, check out lodash as an alternative to underscore - it's got more juice.