Underscore.js 1.7の使い方 Collections編 groupBy

var r = document.getElementById("r");
var result = _.groupBy([1.3, 2.1, 2.4], function(num){
  return 'n' + Math.floor(num);
});
console.log(result);
r.innerHTML = result.n1 + ' ' + result.n2;
// => result = {
//       'n1': [1.3],
//       'n2': [2.1, 2.4]
//    }