Underscore mixin for finding number of occurrences of elements in an array. Optionally select an element to count.

gistfile1.js

/*
  Usage:
  
  _.occurrences(['Foo', 'Bar', 'Foo'])
  => {'Foo': 2, 'Bar': 1}
  
  _.occurrences(['Foo', 'Bar', 'Foo'], 'Foo')
  => {'Foo': 2}
*/
_.mixin({
  occurrences: function(array, needle) {
    return _.reduce(array, function(counter, val) {
      var key = needle || val;
      counter[key] = counter.hasOwnProperty(key) ? counter[key] + 1 : 1;
      return counter;
    }, {});
  }
});