Here is a mix of imperative and functional programming.

imperativeFunk.js

function unflatten(list, group) {  
  var result = [];
  for (var j = 0; j < Math.ceil(list.length/group); j++) {
    for (var i = 0, sublist = [], cache = list[i+(j*group)]; i < group; i++) {
      cache ? sublist.push(cache) : sublist;
    }
    result.push(sublist);
  }
  return result;
}
 
console.log(unflatten(_.range(1, 15), 4));
// [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
 
console.log(_.chain(unflatten(_.range(1, 11), 3))
    // Grab each sublist and...
    .map(function(sublist) {
      // Sum each of its elements up!
      return _.reduce(sublist, function(total, num) {
        return total += num;
      });
    }).value());
// [6, 15, 24, 10]