Fixes #8205. Mitigates memory usage by recycling jsonp callback names the safest possible way (no kittens were harmed in the making of this). Doesn't even try to delete window properties (would necessitate a try/catch for IE which makes the cost in size prohibitive). Unit tests added.

src/ajax/jsonp.js

@@ -1,13 +1,16 @@
 (function( jQuery ) {
 
 var jsc = jQuery.now(),
-	jsre = /(\=)\?(&|$)|\?\?/i;
+	jsre = /(\=)\?(&|$)|\?\?/i,
+	jscallbacks = [];
 
 // Default jsonp settings
 jQuery.ajaxSetup({
 	jsonp: "callback",
 	jsonpCallback: function() {
-		return jQuery.expando + "_" + ( jsc++ );
+		var callback = jscallbacks.pop() || ( jQuery.expando + "_" + ( jsc++ ) );
+		this[ callback ] = true;
+		return callback;
 	}
 });
 
@@ -53,6 +56,13 @@ jQuery.ajaxPrefilter( "json jsonp", function( s, originalSettings, jqXHR ) {
 		jqXHR.always(function() {
 			// Set callback back to previous value
 			window[ jsonpCallback ] = previous;
+			// Save back as free
+			if ( s[ jsonpCallback ] ) {
+				// make sure that re-using the options doesn't screw things around
+				s.jsonpCallback = originalSettings.jsonpCallback;
+				// save the callback name for future use
+				jscallbacks.push( jsonpCallback );
+			}
 			// Call if it was a function and we have a response
 			if ( responseContainer && jQuery.isFunction( previous ) ) {
 				window[ jsonpCallback ]( responseContainer[ 0 ] );

test/unit/ajax.js

@@ -1309,10 +1309,10 @@ test("jQuery.getScript(String, Function) - no callback", function() {
 jQuery.each( [ "Same Domain", "Cross Domain" ] , function( crossDomain , label ) {
 
 	test("jQuery.ajax() - JSONP, " + label, function() {
-		expect(20);
+		expect(24);
 
 		var count = 0;
-		function plus(){ if ( ++count == 18 ) start(); }
+		function plus(){ if ( ++count == 20 ) start(); }
 
 		stop();
 
@@ -1330,6 +1330,25 @@ jQuery.each( [ "Same Domain", "Cross Domain" ] , function( crossDomain , label )
 			}
 		});
 
+		jQuery.ajax({
+			url: "data/jsonp.php",
+			dataType: "jsonp",
+			crossDomain: crossDomain,
+			success: function(data){
+				ok( data.data, ( this.alreadyDone ? "this re-used" : "first request" ) + ": JSON results returned (GET, no callback)" );
+				if ( !this.alreadyDone ) {
+					this.alreadyDone = true;
+					jQuery.ajax( this );
+				} else {
+					plus();
+				}
+			},
+			error: function(data){
+				ok( false, "Ajax error JSON (GET, no callback)" );
+				plus();
+			}
+		});
+
 		jQuery.ajax({
 			url: "data/jsonp.php?callback=?",
 			dataType: "jsonp",
@@ -1564,6 +1583,28 @@ jQuery.each( [ "Same Domain", "Cross Domain" ] , function( crossDomain , label )
 			}
 		});
 
+		//#8205
+		jQuery.ajax({
+			url: "data/jsonp.php",
+			dataType: "jsonp",
+			crossDomain: crossDomain,
+			beforeSend: function() {
+				this.callback = this.jsonpCallback;
+			}
+		}).pipe(function() {
+			var previous = this;
+			strictEqual( previous.jsonpCallback, undefined, "jsonpCallback option is set back to default in callbacks" );
+			jQuery.ajax({
+				url: "data/jsonp.php",
+				dataType: "jsonp",
+				crossDomain: crossDomain,
+				beforeSend: function() {
+					strictEqual( this.jsonpCallback, previous.callback, "JSONP callback name is re-used" );
+					return false;
+				}
+			});
+		}).always( plus );
+
 	});
 });