Mongodb reduce function for complex object

reduce.js

// Universal mongodb reduce function for complex objects
// 
// Example:
//    
//    emit({ total_views: 3, page_type: {home: 1, product: 2}, site_id: [1] })
//    emit({total_views: 5, page_type: {home: 1, collection: 4}, site_id: [2]})
//    
//    -> reduce
//    
//    { total_views: 8, page_type: { home: 2, product: 2, collection: 4 }, site_id: [1,2] }
// 
function(key, values){
  var result = {}

  var merge_obj = function(from, to){
    for (key in from) {
      if (Object.prototype.toString.call(from[key]) == "[object Object]") {
        if (!to[key])
          to[key] = {}

        merge_hash(from[key], to[key])
      } else if (Object.prototype.toString.call(from[key]) == "[object Array]") {
        if (!to[key])
          to[key] = []

        to[key].push(from[key])
      } else {
        if (!to[key])
          to[key] = 0

        to[key] += from[key]
      }
    }
  }

  values.forEach(function(value){
    merge_obj(value, result)
  })

  return result
}