Removed that confusing construct, which had been reduced to only the

'filter' function, and adding more comments.

lib/queue-flow.js

@@ -120,6 +120,10 @@ Q.prototype.handlerCallback = function handlerCallback(openQueueStruct, done) {
     }
 };
 
+// `OpenQueueStruct` is a simple object with fixed types, which should be
+// easier for V8 and other JS engines to optimize internally. `done` is
+// always a boolean and `doneFunc` is always a function. V8 prefers
+// constructor functions to JSON for optimization purposes.
 function OpenQueueStruct() {
     this.done = false;
     this.doneFunc = function() {};
@@ -292,27 +296,6 @@ Q.prototype.wait = function wait(delay) {
 Q.prototype.waitAsync = makeAsync(Q.prototype.wait);
 Q.prototype.waitSync = makeSync(Q.prototype.wait);
 
-// `inOut` is a helper function used by several of the Q prototype methods that
-// take an input queue and produce an output queue.
-var inOut = function inOut(outQueue, setter, callback) {
-    if(isAsync(callback, 2)) {
-        this.setHandlers(function(self, value, struct) {
-            callback(value, function(result) {
-                self.handlerCallback(struct, setter.bind(this, value, result));
-            });
-        }, function(end) {
-            outQueue[end]();
-        });
-    } else {
-        this.setHandlers(function(self, value, struct) {
-            self.handlerCallback(struct, setter.bind(this, value, callback(value)));
-        }, function(end) {
-            outQueue[end]();
-        });
-    }
-    return outQueue;
-};
-
 // `map` creates an output queue, and executes
 // the given callback on each value, pushing the
 // result into the output queue before continuing
@@ -387,10 +370,28 @@ Q.prototype.reduceSync = makeSync(Q.prototype.reduce);
 // original value *only* if the callback returns true.
 Q.prototype.filter = function filter(callback) {
     var outQueue = new Q(null, this.options, this.namespace);
-    return inOut.bind(this)(outQueue, function filterSetter(value, result) {
-        if(result) outQueue.push(value);
-    }, callback);
+    if(isAsync(callback, 2)) {
+        this.setHandlers(function(self, value, struct) {
+            callback(value, function(result) {
+                self.handlerCallback(struct, function() {
+                    if(result) outQueue.push(value);
+                });
+            });
+        }, function(end) {
+            outQueue[end]();
+        });
+    } else {
+        this.setHandlers(function(self, value, struct) {
+            self.handlerCallback(struct, function() {
+                if(callback(value)) outQueue.push(value);
+            });
+        }, function(end) {
+            outQueue[end]();
+        });
+    }
+    return outQueue;
 };
+
 Q.prototype.filterAsync = makeAsync(Q.prototype.filter);
 Q.prototype.filterSync = makeSync(Q.prototype.filter);