diff --git a/lib/DataSet.js b/lib/DataSet.js
index e8d1c973..b87ddcd7 100644
--- a/lib/DataSet.js
+++ b/lib/DataSet.js
@@ -60,15 +60,15 @@ function DataSet (data, options) {
   // all variants of a Date are internally stored as Date, so we can convert
   // from everything to everything (also from ISODate to Number for example)
   if (this._options.type) {
-    for (var field in this._options.type) {
-      if (this._options.type.hasOwnProperty(field)) {
-        var value = this._options.type[field];
-        if (value == 'Date' || value == 'ISODate' || value == 'ASPDate') {
-          this._type[field] = 'Date';
-        }
-        else {
-          this._type[field] = value;
-        }
+    var fields = Object.keys(this._options.type);
+    for (var i = 0, len = fields.length; i < len; i++) {
+      var field = fields[i];
+      var value = this._options.type[field];
+      if (value == 'Date' || value == 'ISODate' || value == 'ASPDate') {
+        this._type[field] = 'Date';
+      }
+      else {
+        this._type[field] = value;
       }
     }
   }
@@ -184,7 +184,7 @@ DataSet.prototype._trigger = function (event, params, senderId) {
     subscribers = subscribers.concat(this._subscribers['*']);
   }
 
-  for (var i = 0; i < subscribers.length; i++) {
+  for (var i = 0, len = subscribers.length; i < len; i++) {
     var subscriber = subscribers[i];
     if (subscriber.callback) {
       subscriber.callback(event, params, senderId || null);
@@ -340,7 +340,7 @@ DataSet.prototype.get = function (args) {
   // build options
   var type = options && options.type || this._options.type;
   var filter = options && options.filter;
-  var items = [], item, itemId, i, len;
+  var items = [], item, itemIds, itemId, i, len;
 
   // convert items
   if (id != undefined) {
@@ -361,12 +361,12 @@ DataSet.prototype.get = function (args) {
   }
   else {
     // return all items
-    for (itemId in this._data) {
-      if (this._data.hasOwnProperty(itemId)) {
-        item = me._getItem(itemId, type);
-        if (!filter || filter(item)) {
-          items.push(item);
-        }
+    itemIds = Object.keys(this._data);
+    for (i = 0, len = itemIds.length; i < len; i++) {
+      itemId = itemIds[i];
+      item = me._getItem(itemId, type);
+      if (!filter || filter(item)) {
+        items.push(item);
       }
     }
   }
@@ -391,9 +391,11 @@ DataSet.prototype.get = function (args) {
 
   // return the results
   if (returnType == 'Object') {
-    var result = {};
-    for (i = 0; i < items.length; i++) {
-      result[items[i].id] = items[i];
+    var result = {},
+        resultant;
+    for (i = 0, len = items.length; i < len; i++) {
+      resultant = items[i];
+      result[resultant.id] = resultant;
     }
     return result;
   }
@@ -422,6 +424,7 @@ DataSet.prototype.getIds = function (options) {
       filter = options && options.filter,
       order = options && options.order,
       type = options && options.type || this._options.type,
+      itemIds = Object.keys(data),
       i,
       len,
       id,
@@ -434,29 +437,27 @@ DataSet.prototype.getIds = function (options) {
     if (order) {
       // create ordered list
       items = [];
-      for (id in data) {
-        if (data.hasOwnProperty(id)) {
-          item = this._getItem(id, type);
-          if (filter(item)) {
-            items.push(item);
-          }
+      for (i = 0, len = itemIds.length; i < len; i++) {
+        id = itemIds[i];
+        item = this._getItem(id, type);
+        if (filter(item)) {
+          items.push(item);
         }
       }
 
       this._sort(items, order);
 
       for (i = 0, len = items.length; i < len; i++) {
-        ids[i] = items[i][this._fieldId];
+        ids.push(items[i][this._fieldId]);
       }
     }
     else {
       // create unordered list
-      for (id in data) {
-        if (data.hasOwnProperty(id)) {
-          item = this._getItem(id, type);
-          if (filter(item)) {
-            ids.push(item[this._fieldId]);
-          }
+      for (i = 0, len = itemIds.length; i < len; i++) {
+        id = itemIds[i];
+        item = this._getItem(id, type);
+        if (filter(item)) {
+          ids.push(item[this._fieldId]);
         }
       }
     }
@@ -466,25 +467,23 @@ DataSet.prototype.getIds = function (options) {
     if (order) {
       // create an ordered list
       items = [];
-      for (id in data) {
-        if (data.hasOwnProperty(id)) {
-          items.push(data[id]);
-        }
+      for (i = 0, len = itemIds.length; i < len; i++) {
+        id = itemIds[i];
+        items.push(data[id]);
       }
 
       this._sort(items, order);
 
       for (i = 0, len = items.length; i < len; i++) {
-        ids[i] = items[i][this._fieldId];
+        ids.push(items[i][this._fieldId]);
       }
     }
     else {
       // create unordered list
-      for (id in data) {
-        if (data.hasOwnProperty(id)) {
-          item = data[id];
-          ids.push(item[this._fieldId]);
-        }
+      for (i = 0, len = itemIds.length; i < len; i++) {
+        id = itemIds[i];
+        item = data[id];
+        ids.push(item[this._fieldId]);
       }
     }
   }
@@ -514,6 +513,9 @@ DataSet.prototype.forEach = function (callback, options) {
   var filter = options && options.filter,
       type = options && options.type || this._options.type,
       data = this._data,
+      itemIds = Object.key(data),
+      i,
+      len,
       item,
       id;
 
@@ -521,7 +523,7 @@ DataSet.prototype.forEach = function (callback, options) {
     // execute forEach on ordered list
     var items = this.get(options);
 
-    for (var i = 0, len = items.length; i < len; i++) {
+    for (i = 0, len = items.length; i < len; i++) {
       item = items[i];
       id = item[this._fieldId];
       callback(item, id);
@@ -529,12 +531,11 @@ DataSet.prototype.forEach = function (callback, options) {
   }
   else {
     // unordered
-    for (id in data) {
-      if (data.hasOwnProperty(id)) {
-        item = this._getItem(id, type);
-        if (!filter || filter(item)) {
-          callback(item, id);
-        }
+    for (i = 0, len = itemIds.length; i < len; i++) {
+      id = itemIds[i];
+      item = this._getItem(id, type);
+      if (!filter || filter(item)) {
+        callback(item, id);
       }
     }
   }
@@ -556,15 +557,18 @@ DataSet.prototype.map = function (callback, options) {
       type = options && options.type || this._options.type,
       mappedItems = [],
       data = this._data,
+      itemIds = Object.keys(data),
+      i,
+      len,
+      id,
       item;
 
   // convert and filter items
-  for (var id in data) {
-    if (data.hasOwnProperty(id)) {
-      item = this._getItem(id, type);
-      if (!filter || filter(item)) {
-        mappedItems.push(callback(item, id));
-      }
+  for (i = 0, len = itemIds.length; i < len; i++) {
+    id = itemIds[i];
+    item = this._getItem(id, type);
+    if (!filter || filter(item)) {
+      mappedItems.push(callback(item, id));
     }
   }
 
@@ -588,17 +592,23 @@ DataSet.prototype._filterFields = function (item, fields) {
     return item;
   }
 
-  var filteredItem = {};
+  var filteredItem = {},
+      itemFields = Object.keys(item),
+      len = itemFields.length,
+      i,
+      field;
 
   if(Array.isArray(fields)){
-    for (var field in item) {
-      if (item.hasOwnProperty(field) && (fields.indexOf(field) != -1)) {
+    for (i = 0; i < len; i++) {
+      field = itemFields[i];
+      if (fields.indexOf(field) != -1) {
         filteredItem[field] = item[field];
       }
     }
   }else{
-    for (var field in item) {
-      if (item.hasOwnProperty(field) && fields.hasOwnProperty(field)) {
+    for (i = 0; i < len; i++) {
+      field = itemFields[i];
+      if (fields.hasOwnProperty(field)) {
         filteredItem[fields[field]] = item[field];
       }
     }
@@ -715,17 +725,19 @@ DataSet.prototype.clear = function (senderId) {
  */
 DataSet.prototype.max = function (field) {
   var data = this._data,
+      itemIds = Object.keys(data),
       max = null,
-      maxField = null;
-
-  for (var id in data) {
-    if (data.hasOwnProperty(id)) {
-      var item = data[id];
-      var itemField = item[field];
-      if (itemField != null && (!max || itemField > maxField)) {
-        max = item;
-        maxField = itemField;
-      }
+      maxField = null,
+      i,
+      len;
+
+  for (i = 0, len = itemIds.length; i < len; i++) {
+    var id = itemIds[i];
+    var item = data[id];
+    var itemField = item[field];
+    if (itemField != null && (!max || itemField > maxField)) {
+      max = item;
+      maxField = itemField;
     }
   }
 
@@ -739,17 +751,19 @@ DataSet.prototype.max = function (field) {
  */
 DataSet.prototype.min = function (field) {
   var data = this._data,
+      itemIds = Object.keys(data),
       min = null,
-      minField = null;
-
-  for (var id in data) {
-    if (data.hasOwnProperty(id)) {
-      var item = data[id];
-      var itemField = item[field];
-      if (itemField != null && (!min || itemField < minField)) {
-        min = item;
-        minField = itemField;
-      }
+      minField = null,
+      i,
+      len;
+
+  for (i = 0, len = itemIds.length; i < len; i++) {
+    var id = itemIds[i];
+    var item = data[id];
+    var itemField = item[field];
+    if (itemField != null && (!min || itemField < minField)) {
+      min = item;
+      minField = itemField;
     }
   }
 
@@ -765,31 +779,33 @@ DataSet.prototype.min = function (field) {
  */
 DataSet.prototype.distinct = function (field) {
   var data = this._data;
+  var itemIds = Object.keys(data);
   var values = [];
   var fieldType = this._options.type && this._options.type[field] || null;
   var count = 0;
-  var i;
-
-  for (var prop in data) {
-    if (data.hasOwnProperty(prop)) {
-      var item = data[prop];
-      var value = item[field];
-      var exists = false;
-      for (i = 0; i < count; i++) {
-        if (values[i] == value) {
-          exists = true;
-          break;
-        }
-      }
-      if (!exists && (value !== undefined)) {
-        values[count] = value;
-        count++;
+  var i,
+      j,
+      len;
+
+  for (i = 0, len = itemIds.length; i < len; i++) {
+    var id = itemIds[i];
+    var item = data[id];
+    var value = item[field];
+    var exists = false;
+    for (j = 0; j < count; j++) {
+      if (values[j] == value) {
+        exists = true;
+        break;
       }
     }
+    if (!exists && (value !== undefined)) {
+      values[count] = value;
+      count++;
+    }
   }
 
   if (fieldType) {
-    for (i = 0; i < values.length; i++) {
+    for (i = 0, len = values.length; i < len; i++) {
       values[i] = util.convert(values[i], fieldType);
     }
   }
@@ -819,12 +835,14 @@ DataSet.prototype._addItem = function (item) {
     item[this._fieldId] = id;
   }
 
-  var d = {};
-  for (var field in item) {
-    if (item.hasOwnProperty(field)) {
-      var fieldType = this._type[field];  // type may be undefined
-      d[field] = util.convert(item[field], fieldType);
-    }
+  var d = {},
+      fields = Object.keys(item),
+      i,
+      len;
+  for (i = 0, len = fields.length; i < len; i++) {
+    var field = fields[i];
+    var fieldType = this._type[field];  // type may be undefined
+    d[field] = util.convert(item[field], fieldType);
   }
   this._data[id] = d;
   this.length++;
@@ -840,7 +858,7 @@ DataSet.prototype._addItem = function (item) {
  * @private
  */
 DataSet.prototype._getItem = function (id, types) {
-  var field, value;
+  var field, value, i, len;
 
   // get the item from the dataset
   var raw = this._data[id];
@@ -849,22 +867,22 @@ DataSet.prototype._getItem = function (id, types) {
   }
 
   // convert the items field types
-  var converted = {};
+  var converted = {},
+      fields = Object.keys(raw);
+
   if (types) {
-    for (field in raw) {
-      if (raw.hasOwnProperty(field)) {
-        value = raw[field];
-        converted[field] = util.convert(value, types[field]);
-      }
+    for (i = 0, len = fields.length; i < len; i++) {
+      field = fields[i];
+      value = raw[field];
+      converted[field] = util.convert(value, types[field]);
     }
   }
   else {
     // no field types specified, no converting needed
-    for (field in raw) {
-      if (raw.hasOwnProperty(field)) {
-        value = raw[field];
-        converted[field] = value;
-      }
+    for (i = 0, len = fields.length; i < len; i++) {
+      field = fields[i];
+      value = raw[field];
+      converted[field] = value;
     }
   }
   return converted;
@@ -890,11 +908,11 @@ DataSet.prototype._updateItem = function (item) {
   }
 
   // merge with current item
-  for (var field in item) {
-    if (item.hasOwnProperty(field)) {
-      var fieldType = this._type[field];  // type may be undefined
-      d[field] = util.convert(item[field], fieldType);
-    }
+  var fields = Object.keys(item);
+  for (var i = 0, len = fields.length; i < len; i++) {
+    var field = fields[i];
+    var fieldType = this._type[field];  // type may be undefined
+    d[field] = util.convert(item[field], fieldType);
   }
 
   return id;
diff --git a/lib/DataView.js b/lib/DataView.js
index 39da84ab..7d5b2316 100644
--- a/lib/DataView.js
+++ b/lib/DataView.js
@@ -35,7 +35,7 @@ function DataView (data, options) {
  * @param {DataSet | DataView} data
  */
 DataView.prototype.setData = function (data) {
-  var ids, i, len;
+  var ids, id, i, len;
 
   if (this._data) {
     // unsubscribe from current dataset
@@ -44,12 +44,7 @@ DataView.prototype.setData = function (data) {
     }
 
     // trigger a remove of all items in memory
-    ids = [];
-    for (var id in this._ids) {
-      if (this._ids.hasOwnProperty(id)) {
-        ids.push(id);
-      }
-    }
+    ids = Object.keys(this._ids);
     this._ids = {};
     this.length = 0;
     this._trigger('remove', {items: ids});
@@ -84,34 +79,34 @@ DataView.prototype.setData = function (data) {
  * containing a variable parameter.
  */
 DataView.prototype.refresh = function () {
-  var id;
+  var id, i, len;
   var ids = this._data.getIds({filter: this._options && this._options.filter});
+  var oldIds = Object.keys(this._ids);
   var newIds = {};
   var added = [];
   var removed = [];
 
   // check for additions
-  for (var i = 0; i < ids.length; i++) {
+  for (i = 0, len = ids.length; i < len; i++) {
     id = ids[i];
     newIds[id] = true;
     if (!this._ids[id]) {
       added.push(id);
       this._ids[id] = true;
-      this.length++;
     }
   }
 
   // check for removals
-  for (id in this._ids) {
-    if (this._ids.hasOwnProperty(id)) {
-      if (!newIds[id]) {
-        removed.push(id);
-        delete this._ids[id];
-        this.length--;
-      }
+  for (i = 0, len = oldIds.length; i < len; i++) {
+    id = oldIds[i];
+    if (!newIds[id]) {
+      removed.push(id);
+      delete this._ids[id];
     }
   }
 
+  this.length += added.length - removed.length;
+
   // trigger events
   if (added.length) {
     this._trigger('add', {items: added});