groupByMulti.js

function groupBy(array, i) {
  var groups = {};
  array.forEach(function(o) {
    var group = o[i];
    groups[group] = groups[group] || [];
    groups[group].push(o);
  });
  return groups
}

function groupByMulti(list, values) {
  if (!values.length) {
    return list;
  }
  var byFirst = groupBy(list, values[0]);
  var rest    = values.slice(1);
  for (var prop in byFirst) {
    byFirst[prop] = groupByMulti(byFirst[prop], rest);
  }
  return byFirst;
}