Fixes #2994. Not finding a transport now fires the error callbacks and doesn't make ajax return false. Had to revise how jsonp and script prefilters & transports work (better separation of concerns). Also took the opportunity to revise jXHR getRequestHeader and abort methods and enabled early transport garbage collection when the request completes.

src/ajax.js

@@ -306,9 +306,7 @@ jQuery.extend({
 				// (match is used internally)
 				getResponseHeader: function( key , match ) {
 
-					if ( state !== 2 ) {
-						return null;
-					}
+					if ( state === 2 ) {
 
 						if ( responseHeaders === undefined ) {
 
@@ -321,15 +319,22 @@ jQuery.extend({
 								}
 							}
 						}
-					return responseHeaders[ key.toLowerCase() ];
+						match = responseHeaders[ key.toLowerCase() ];
+
+					} else {
+
+						match = null;
+					}
+
+					return match;
 				},
 
 				// Cancel the request
 				abort: function( statusText ) {
-					if ( transport && state !== 2 ) {
+					if ( transport ) {
 						transport.abort( statusText || "abort" );
-						done( 0 , statusText );
 					}
+					done( 0 , statusText );
 					return this;
 				}
 			};
@@ -347,6 +352,10 @@ jQuery.extend({
 			// State is "done" now
 			state = 2;
 
+			// Dereference transport for early garbage collection
+			// (no matter how long the jXHR transport will be used
+			transport = 0;
+
 			// Set readyState
 			jXHR.readyState = status ? 4 : 0;
 
@@ -599,22 +608,14 @@ jQuery.extend({
 			s.data = jQuery.param( s.data , s.traditional );
 		}
 
-		// Get transport
-		transport = jQuery.ajaxPrefilter( s , options ).ajaxTransport( s );
+		// Apply prefilters
+		jQuery.ajaxPrefilter( s , options );
 
 		// Watch for a new set of requests
 		if ( s.global && jQuery.active++ === 0 ) {
 			jQuery.event.trigger( "ajaxStart" );
 		}
 
-		// If no transport, we auto-abort
-		if ( ! transport ) {
-
-			done( 0 , "transport not found" );
-			jXHR = false;
-
-		} else {
-
 		// More options handling for requests with no content
 		if ( ! s.hasContent ) {
 
@@ -665,19 +666,30 @@ jQuery.extend({
 
 				// Abort if not done already
 				done( 0 , "abort" );
+
+				// Return false
 				jXHR = false;
 
 		} else {
 
-				// Set state as sending
-				state = 1;
-				jXHR.readyState = 1;
-
 			// Install callbacks on deferreds
 			for ( i in { success:1, error:1, complete:1 } ) {
 				jXHR[ i ]( s[ i ] );
 			}
 
+			// Get transport
+			transport = jQuery.ajaxTransport( s );
+
+			// If no transport, we auto-abort
+			if ( ! transport ) {
+
+				done( 0 , "notransport" );
+
+			} else {
+
+				// Set state as sending
+				state = jXHR.readyState = 1;
+
 				// Send global event
 				if ( s.global ) {
 					globalEventContext.trigger( "ajaxSend" , [ jXHR , s ] );

src/ajax/jsonp.js

@@ -11,9 +11,7 @@ jQuery.ajaxSetup({
 		return "jsonp" + jsc++;
 	}
 
-// Normalize jsonp queries
-// 1) put callback parameter in url or data
-// 2) sneakily ensure transportDataType is always jsonp for jsonp requests
+// Detect, normalize options and install callbacks for jsonp requests
 }).ajaxPrefilter("json jsonp", function(s, originalSettings) {
 
 	if ( s.dataTypes[ 0 ] === "jsonp" ||
@@ -22,8 +20,10 @@ jQuery.ajaxSetup({
 		jsre.test(s.url) ||
 		typeof(s.data) === "string" && jsre.test(s.data) ) {
 
-		var jsonpCallback = s.jsonpCallback =
+		var responseContainer,
+			jsonpCallback = s.jsonpCallback =
 				jQuery.isFunction( s.jsonpCallback ) ? s.jsonpCallback() : s.jsonpCallback,
+			previous = window[ jsonpCallback ],
 			url = s.url.replace(jsre, "$1" + jsonpCallback + "$2"),
 			data = s.url === url && typeof(s.data) === "string" ? s.data.replace(jsre, "$1" + jsonpCallback + "$2") : s.data;
 
@@ -33,16 +33,6 @@ jQuery.ajaxSetup({
 
 		s.url = url;
 		s.data = data;
-		s.dataTypes[ 0 ] = "jsonp";
-	}
-
-// Bind transport to jsonp dataType
-}).ajaxTransport("jsonp", function(s) {
-
-	// Put callback in place
-	var responseContainer,
-		jsonpCallback = s.jsonpCallback,
-		previous = window[ jsonpCallback ];
 
 		window [ jsonpCallback ] = function( response ) {
 			responseContainer = [response];
@@ -65,9 +55,6 @@ jQuery.ajaxSetup({
 
 		}, s.complete ];
 
-	// Sneakily ensure this will be handled as json
-	s.dataTypes[ 0 ] = "json";
-
 		// Use data converter to retrieve json after script execution
 		s.converters["script json"] = function() {
 			if ( ! responseContainer ) {
@@ -76,8 +63,12 @@ jQuery.ajaxSetup({
 			return responseContainer[ 0 ];
 		};
 
-	// Delegate to script transport
+		// force json dataType
+		s.dataTypes[ 0 ] = "json";
+
+		// Delegate to script
 		return "script";
+	}
 });
 
 })( jQuery );

src/ajax/script.js

@@ -15,18 +15,22 @@ jQuery.ajaxSetup({
 		"text script": jQuery.globalEval
 	}
 
-// Bind script tag hack transport
-}).ajaxTransport("script", function(s) {
+// Handle cache's special case and global
+}).ajaxPrefilter("script", function(s) {
 
-	// Handle cache special case
 	if ( s.cache === undefined ) {
 		s.cache = false;
 	}
 
-	// This transport only deals with cross domain get requests
-	if ( s.crossDomain && s.async && ( s.type === "GET" || ! s.data ) ) {
-
+	if ( s.crossDomain ) {
 		s.global = false;
+	}
+
+// Bind script tag hack transport
+}).ajaxTransport("script", function(s) {
+
+	// This transport only deals with cross domain requests
+	if ( s.crossDomain ) {
 
 		var script,
 			head = document.getElementsByTagName("head")[0] || document.documentElement;

test/unit/ajax.js

@@ -1865,25 +1865,19 @@ test("jQuery ajax - failing cross-domain", function() {
 
 	var i = 2;
 
-	if ( jQuery.ajax({
+	jQuery.ajax({
 		url: 'http://somewebsitethatdoesnotexist-67864863574657654.com',
 		success: function(){ ok( false , "success" ); },
 		error: function(xhr,_,e){ ok( true , "file not found: " + xhr.status + " => " + e ); },
 		complete: function() { if ( ! --i ) start(); }
-	}) === false ) {
-		ok( true , "no transport" );
-		if ( ! --i ) start();
-	}
+	});
 
-	if ( jQuery.ajax({
+	jQuery.ajax({
 		url: 'http://www.google.com',
 		success: function(){ ok( false , "success" ); },
 		error: function(xhr,_,e){ ok( true , "access denied: " + xhr.status + " => " + e ); },
 		complete: function() { if ( ! --i ) start(); }
-	}) === false ) {
-		ok( true , "no transport" );
-		if ( ! --i ) start();
-	}
+	});
 
 });
 
@@ -1937,7 +1931,7 @@ test( "jQuery.ajax - statusCode" , function() {
 			404: function() {
 				ok( ! isSuccess , name );
 			}
-		}
+		};
 	}
 
 	jQuery.each( {