Using Array.prototype.reduce; Given an array of values, returns array of the sums of the values per INTERVAL.

interval-sum.js

var INTERVAL = 5;
var list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11];

list.reduce(function(result, value, index) {
  var i = Math.floor(index/INTERVAL);
  result[i] ? result[i] += value : result[i] = value;
  return result;
}, []);

// Output: [15, 40, 11]

It was at what i was looking for and worked perfect!!!