Underscore.js 1.7の使い方 Collections編 countBy

var r = document.getElementById("r");
var result = _.countBy([1, 2, 3, 4, 5], function(num) {
  return num % 2 === 1 ? 'odd': 'even';
});
// => result = { odd: 3, even: 2 }
r.innerHTML = result.odd + ' ' + result.even;
// => 3 2