diff --git a/underscore.js b/underscore.js
index d76efb223..e3f938ba1 100644
--- a/underscore.js
+++ b/underscore.js
@@ -43,11 +43,10 @@
       if (obj.forEach) {
         obj.forEach(iterator, context);
       } else if (obj.length) {
-        for (var i=0, l = obj.length; i<l; i++) iterator.call(context, obj[i], i, obj);
+        for (var i=0, l=obj.length; i<l; i++) iterator.call(context, obj[i], i, obj);
       } else {
-        for (var key in obj) if (Object.prototype.hasOwnProperty.call(obj, key)) {
-          iterator.call(context, obj[key], key, obj);
-        }
+        var keys = _.keys(obj), l = keys.length;
+        for (var i=0; i<l; i++) iterator.call(context, obj[keys[i]], keys[i], obj);
       }
     } catch(e) {
       if (e != breaker) throw e;
@@ -391,7 +390,10 @@
 
   // Retrieve the names of an object's properties.
   _.keys = function(obj) {
-    return _.map(obj, function(value, key){ return key; });
+    if(_.isArray(obj)) return _.range(0, obj.length);
+    var keys = [];
+    for (var key in obj) if (Object.prototype.hasOwnProperty.call(obj, key)) keys.push(key);
+    return keys;
   };
 
   // Retrieve the values of an object's properties.
@@ -441,7 +443,7 @@
 
   // Is a given array or object empty?
   _.isEmpty = function(obj) {
-    return (_.isArray(obj) ? obj : _.values(obj)).length == 0;
+    return _.keys(obj).length == 0;
   };
 
   // Is a given value a DOM element?
@@ -503,9 +505,7 @@
 
   // Return a sorted list of the function names available in Underscore.
   _.functions = function() {
-    var functions = [];
-    for (var key in _) if (Object.prototype.hasOwnProperty.call(_, key)) functions.push(key);
-    return _.without(functions, 'VERSION', 'prototype', 'noConflict').sort();
+    return _.without(_.keys(_), 'VERSION', 'prototype', 'noConflict').sort();
   };
 
   // JavaScript templating a-la ERB, pilfered from John Resig's