Ajax: Remove jsonp callbacks through "jQuery#removeProp" method

Fixes gh-2323
Closes gh-2464
Ref a2ae215d999637e8d9d0906abcbf6b1ca35c8e6e

src/ajax/jsonp.js

@@ -64,8 +64,14 @@ jQuery.ajaxPrefilter( "json jsonp", function( s, originalSettings, jqXHR ) {
 
 		// Clean-up function (fires after converters)
 		jqXHR.always(function() {
-			// Restore preexisting value
+			// If previous value didn't exist - remove it
+			if ( overwritten === undefined ) {
+				jQuery( window ).removeProp( callbackName );
+
+			// Otherwise restore preexisting value
+			} else {
 				window[ callbackName ] = overwritten;
+			}
 
 			// Save back as free
 			if ( s[ callbackName ] ) {

test/unit/ajax.js

+var isIE8 = /msie 8\.0/i.test( window.navigator.userAgent );
+
 module( "ajax", {
 	setup: function() {
+		if ( !isIE8 ) {
+			return;
+		}
+
 		var jsonpCallback = this.jsonpCallback = jQuery.ajaxSettings.jsonpCallback;
 		jQuery.ajaxSettings.jsonpCallback = function() {
 			var callback = jsonpCallback.apply( this, arguments );
@@ -737,7 +743,7 @@ module( "ajax", {
 			}
 		]);
 
-		ajaxTest( "jQuery.ajax() - JSONP - Explicit callback param" + label, 9, {
+		ajaxTest( "jQuery.ajax() - JSONP - Explicit callback param" + label, 10, {
 			setup: function() {
 				Globals.register("functionToCleanUp");
 				Globals.register("XXX");
@@ -760,6 +766,11 @@ module( "ajax", {
 				crossDomain: crossDomain,
 				jsonpCallback: "jsonpResults",
 				success: function( data ) {
+					strictEqual(
+						typeof window[ "jsonpResults" ],
+						"function",
+						"should not rewrite original function"
+					);
 					ok( data.data, "JSON results returned (GET, custom callback name)" );
 				}
 			}, {
@@ -1372,16 +1383,34 @@ module( "ajax", {
 	]);
 
 	jQuery.each( [ " - Same Domain", " - Cross Domain" ], function( crossDomain, label ) {
-		ajaxTest( "#8205 - jQuery.ajax() - JSONP - re-use callbacks name" + label, 2, {
+		ajaxTest( "#8205 - jQuery.ajax() - JSONP - re-use callbacks name" + label, 4, {
 			url: "data/jsonp.php",
 			dataType: "jsonp",
 			crossDomain: crossDomain,
 			beforeSend: function( jqXHR, s ) {
 				s.callback = s.jsonpCallback;
+
+				ok( this.callback in window, "JSONP callback name is in the window" );
 			},
 			success: function() {
 				var previous = this;
-				strictEqual( previous.jsonpCallback, undefined, "jsonpCallback option is set back to default in callbacks" );
+
+				strictEqual(
+					previous.jsonpCallback,
+					undefined,
+					"jsonpCallback option is set back to default in callbacks"
+				);
+
+				if ( isIE8 ) {
+					ok( true, "IE8 can't remove property from the window" );
+
+				} else {
+					ok(
+						!( this.callback in window ),
+						"JSONP callback name was removed from the window"
+					);
+				}
+
 				jQuery.ajax({
 					url: "data/jsonp.php",
 					dataType: "jsonp",