diff --git a/lib/async.js b/lib/async.js
index dec593980..314432e91 100644
--- a/lib/async.js
+++ b/lib/async.js
@@ -107,11 +107,24 @@
         return keys;
     };
 
-    function _iteratorKeys(coll) {
-        return _isArrayLike(coll) ?
-            // just plain _keys wont work with sparse arrays
-            _map(coll, function (_, index) { return index; }) :
-            _keys(coll);
+    function _keyIterator(coll) {
+        var i = -1;
+        var len;
+        var keys;
+        if (_isArrayLike(coll)) {
+            len = coll.length;
+            return function next() {
+                i++;
+                return i < len ? i : null;
+            };
+        } else {
+            keys = _keys(coll);
+            len = keys.length;
+            return function next() {
+                i++;
+                return i < len ? keys[i] : null;
+            };
+        }
     }
 
     function _baseSlice(arr, start) {
@@ -219,32 +232,24 @@
     async.forEachOfSeries =
     async.eachOfSeries = function (obj, iterator, callback) {
         callback = callback || noop;
-        var keys = _iteratorKeys(obj);
-        var size = keys.length;
-        if (!size) {
-            return callback();
-        }
-        var completed = 0;
+        var nextKey = _keyIterator(obj);
         function iterate() {
             var sync = true;
-            var key = keys[completed];
+            var key = nextKey();
+            if (key === null) {
+                return callback(null);
+            }
             iterator(obj[key], key, function (err) {
                 if (err) {
                     callback(err);
                     callback = noop;
                 }
                 else {
-                    completed += 1;
-                    if (completed >= size) {
-                        callback(null);
+                    if (sync) {
+                        async.nextTick(iterate);
                     }
                     else {
-                        if (sync) {
-                            async.nextTick(iterate);
-                        }
-                        else {
-                            iterate();
-                        }
+                        iterate();
                     }
                 }
             });
@@ -264,40 +269,41 @@
 
         return function (obj, iterator, callback) {
             callback = callback || noop;
-            var keys = _iteratorKeys(obj);
-            var size = keys.length;
-            if (!size || limit <= 0) {
+            var nextKey = _keyIterator(obj);
+            if (limit <= 0) {
                 return callback(null);
             }
-            var completed = 0;
-            var started = 0;
+            var done = false;
             var running = 0;
             var errored = false;
 
             (function replenish () {
-                if (completed >= size) {
-                    return callback();
+                if (done && running <= 0) {
+                    callback(null);
+                    callback = noop;
+                    return;
                 }
 
-                while (running < limit && started < size && !errored) {
-                    started += 1;
+                while (running < limit && !errored) {
+                    var key = nextKey();
+                    if (key === null) {
+                        done = true;
+                        if (running <= 0) {
+                            callback(null);
+                            callback = noop;
+                        }
+                        return;
+                    }
                     running += 1;
-                    var key = keys[started - 1];
                     iterator(obj[key], key, function (err) {
+                        running -= 1;
                         if (err) {
                             callback(err);
                             errored = true;
                             callback = noop;
                         }
                         else {
-                            completed += 1;
-                            running -= 1;
-                            if (completed >= size) {
-                                callback(null);
-                            }
-                            else {
-                                replenish();
-                            }
+                            replenish();
                         }
                     });
                 }