This is the exact, 100% correct behavior of $.extend

Replying to [comment:1 rwaldron]:

This is the exact, 100% correct behavior of $.extend

I'm sorry, perhaps I wasn't clear. I'm not suggesting that extend is misbehaving. I agree that the extend behavior is correct. What I am suggesting is that because of this behavior, extend, on its own, is not suitable for merging the ajax data setting.

The bug is that data is allowed to be an object or a string, and this is clearly documented, but that the behavior is different in each case, which is not documented.

Consider this example:

$.ajaxSetup( { data: { foo: 1 } } )

// These requests should be identical, but they are not
$('#id1').load( url, {bar:2}, callback )
$('#id2').load( url, "bar=2", callback )

I don't see any way to attach a file (sorry if I'm missing something). Here is a proposed patch that solves the problem.

diff --git a/jquery-1.5.2.js b/jquery-1.5.2.js
--- a/jquery-1.5.2.js
+++ b/jquery-1.5.2.js
@@ -6383,14 +6383,31 @@
        // with both ajaxSettings and settings fields.
        // If target is omitted, writes into ajaxSettings.
        ajaxSetup: function ( target, settings ) {
+               var data_args = [];
                if ( !settings ) {
                        // Only one parameter, we extend ajaxSettings
                        settings = target;
                        target = jQuery.extend( true, jQuery.ajaxSettings, settings );
+                       data_args.push( settings.data, jQuery.ajaxSettings.data );
                } else {
                        // target was provided, we extend into it
                        jQuery.extend( true, target, jQuery.ajaxSettings, settings );
-               }
+                       data_args.push( settings.data, jQuery.ajaxSettings.data, target.data );
+               }
+
+               // Make sure already-serialized data gets merged correctly
+               if ( target.processData ) {
+                       var data_strings = jQuery.map( data_args, function( data, index ) {
+                               if ( data == null )
+                                       return null;
+                               else if ( typeof data === "string" )
+                                       return data;
+                               else
+                                       return jQuery.param( data, target.traditional );
+                       });
+                       target.data = data_strings.join( '&' )
+               }
+
                // Flatten fields we don't want deep extended
                for( var field in { context: 1, url: 1 } ) {
                        if ( field in settings ) {
@@ -6712,11 +6729,6 @@
                        );
                }
 
-               // Convert data if not already a string
-               if ( s.data && s.processData && typeof s.data !== "string" ) {
-                       s.data = jQuery.param( s.data, s.traditional );
-               }
-
                // Apply prefilters
                inspectPrefiltersOrTransports( prefilters, s, options, jqXHR );

I don't think that's something we want to change. Data is correctly overwritten by the second.

Why not just do:

$.extend({}, { data: "foo=1"+"bar=2" });

or some equivalent.

Replying to [comment:2 danielh@…]:

Replying to [comment:1 rwaldron]: > This is the exact, 100% correct behavior of $.extend > I'm sorry, perhaps I wasn't clear. I'm not suggesting that extend is misbehaving. I agree that the extend behavior is correct. What I am suggesting is that because of this behavior, extend, on its own, is not suitable for merging the ajax data setting. The bug is that data is allowed to be an object or a string, and this is clearly documented, but that the behavior is different in each case, which is not documented. Consider this example:

> 
> $.ajaxSetup( { data: { foo: 1 } } )
> 
> // These requests should be identical, but they are not
> $('#id1').load( url, {bar:2}, callback )
> $('#id2').load( url, "bar=2", callback )
> 
> 

I don't see any way to attach a file (sorry if I'm missing something). Here is a proposed patch that solves the problem.

> 
> diff --git a/jquery-1.5.2.js b/jquery-1.5.2.js
> --- a/jquery-1.5.2.js
> +++ b/jquery-1.5.2.js
> @@ -6383,14 +6383,31 @@
>         // with both ajaxSettings and settings fields.
>         // If target is omitted, writes into ajaxSettings.
>         ajaxSetup: function ( target, settings ) {
> +               var data_args = [];
>                 if ( !settings ) {
>                         // Only one parameter, we extend ajaxSettings
>                         settings = target;
>                         target = jQuery.extend( true, jQuery.ajaxSettings, settings );
> +                       data_args.push( settings.data, jQuery.ajaxSettings.data );
>                 } else {
>                         // target was provided, we extend into it
>                         jQuery.extend( true, target, jQuery.ajaxSettings, settings );
> -               }
> +                       data_args.push( settings.data, jQuery.ajaxSettings.data, target.data );
> +               }
> +
> +               // Make sure already-serialized data gets merged correctly
> +               if ( target.processData ) {
> +                       var data_strings = jQuery.map( data_args, function( data, index ) {
> +                               if ( data == null )
> +                                       return null;
> +                               else if ( typeof data === "string" )
> +                                       return data;
> +                               else
> +                                       return jQuery.param( data, target.traditional );
> +                       });
> +                       target.data = data_strings.join( '&' )
> +               }
> +
>                 // Flatten fields we don't want deep extended
>                 for( var field in { context: 1, url: 1 } ) {
>                         if ( field in settings ) {
> @@ -6712,11 +6729,6 @@
>                         );
>                 }
>  
> -               // Convert data if not already a string
> -               if ( s.data && s.processData && typeof s.data !== "string" ) {
> -                       s.data = jQuery.param( s.data, s.traditional );
> -               }
> -
>                 // Apply prefilters
>                 inspectPrefiltersOrTransports( prefilters, s, options, jqXHR );
> 
> 

Your code assumes the data is url encoded. What if it's a json or xml document?

Truth is you can't know for sure what the intent is when data is given as string and serializing like you do is not a solution and will actually break existing applications.

I like the overall idea, mind you, but I'm unsure of its merits on a generality point of view.