Add hook for modifying the merge behavior of $.extend. Specifically, and time there is a collision between the target and mergee this function is called to resolve it.

src/jquery/jquery.js

@@ -1178,6 +1178,10 @@ jQuery.fn = jQuery.prototype = {
  * Extend one object with one or more others, returning the original,
  * modified, object. This is a great utility for simple inheritance.
  *
+ * There is also an optional collision resolution function. Any time the target and
+ * merged object both contain a key this function is called. This may be used to create
+ * a recursive merge. (See example) Unless you know what this is, you probably don't care.
+ * 
  * @example var settings = { validate: false, limit: 5, name: "foo" };
  * var options = { validate: true, name: "bar" };
  * jQuery.extend(settings, options);
@@ -1190,8 +1194,19 @@ jQuery.fn = jQuery.prototype = {
  * @result settings == { validate: true, limit: 5, name: "bar" }
  * @desc Merge defaults and options, without modifying the defaults
  *
+ * @example var defaults = { validate: false, limit: 5, name: "foo", nested: {depth: false} };
+ * var options = { validate: true, name: "bar", nested: {depth: true} };
+ * var collision_resolver_fn = function(target, mergee) {
+ *   // combine nested Objects, in this case the object being merged takes priority
+ *   return jQuery.extend({}, target, mergee);
+ * }
+ * var settings = jQuery.extend({}, collision_resolver_fn, defaults, options);
+ * @result settings == { validate: true, limit: 5, name: "bar" }
+ * @desc Recursively merge defaults and options, without modifying the defaults
+ *
  * @name $.extend
  * @param Object target The object to extend
+ * @param Function (optional) collision resolution function. Hook to extend the merging behavior of $.extend(). See example.
  * @param Object prop1 The object that will be merged into the first.
  * @param Object propN (optional) More objects to merge into the first
  * @type Object
@@ -1199,18 +1214,26 @@ jQuery.fn = jQuery.prototype = {
  */
 jQuery.extend = jQuery.fn.extend = function() {
 	// copy reference to target object
-	var target = arguments[0],
+	var resolver, prop, target = arguments[0],
 		a = 1;
 	
 	// extend jQuery itself if only one argument is passed
 	if ( arguments.length == 1 ) {
 		target = this;
 		a = 0;
+	} else if (jQuery.isFunction(arguments[a])) {
+	  resolver = arguments[a++];
 	}
-	var prop;
+
 	while (prop = arguments[a++])
 		// Extend the base object
-		for ( var i in prop ) target[i] = prop[i];
+		for ( var i in prop ) {
+		  if (resolver && target[i] && prop[i]) {
+		    target[i] = resolver(target[i], prop[i]);
+	    } else {
+		    target[i] = prop[i];
+		  }
+	  }
 
 	// Return the modified object
 	return target;