Makes sure no unload handler is bound when not in IE. Also simplifies the whole "on unload abort" code. Also avoids the declaration of yet another variables in the jQuery main closure for the temporary XHR used to assess support properties.

Author: jaubourg

src/ajax/xhr.js

@@ -1,21 +1,14 @@
 (function( jQuery ) {
 
-var // #5280: next active xhr id and list of active xhrs' callbacks
-	xhrId = jQuery.now(),
-	xhrCallbacks,
-
-	// XHR used to determine supports properties
-	testXHR;
-
-// #5280: Internet Explorer will keep connections alive if we don't abort on unload
-function xhrOnUnloadAbort() {
-	jQuery( window ).unload(function() {
+var // #5280: Internet Explorer will keep connections alive if we don't abort on unload
+	xhrOnUnloadAbort = window.ActiveXObject ? function() {
 		// Abort all pending requests
 		for ( var key in xhrCallbacks ) {
 			xhrCallbacks[ key ]( 0, 1 );
 		}
-	});
-}
+	} : false,
+	xhrId = 0,
+	xhrCallbacks;
 
 // Functions to create xhrs
 function createStandardXHR() {
@@ -45,15 +38,13 @@ jQuery.ajaxSettings.xhr = window.ActiveXObject ?
 	// For all other browsers, use the standard XMLHttpRequest object
 	createStandardXHR;
 
-// Test if we can create an xhr object
-testXHR = jQuery.ajaxSettings.xhr();
-jQuery.support.ajax = !!testXHR;
-
-// Does this browser support crossDomain XHR requests
-jQuery.support.cors = testXHR && ( "withCredentials" in testXHR );
-
-// No need for the temporary xhr anymore
-testXHR = undefined;
+// Determine support properties
+(function( xhr ) {
+	jQuery.extend( jQuery.support, {
+		ajax: !!xhr,
+		cors: !!xhr && ( "withCredentials" in xhr )
+	});
+})( jQuery.ajaxSettings.xhr() );
 
 // Create transport if the browser can provide an xhr
 if ( jQuery.support.ajax ) {
@@ -136,7 +127,9 @@ if ( jQuery.support.ajax ) {
 								// Do not keep as active anymore
 								if ( handle ) {
 									xhr.onreadystatechange = jQuery.noop;
-									delete xhrCallbacks[ handle ];
+									if ( xhrOnUnloadAbort ) {
+										delete xhrCallbacks[ handle ];
+									}
 								}
 
 								// If it's an abort
@@ -197,15 +190,18 @@ if ( jQuery.support.ajax ) {
 					if ( !s.async || xhr.readyState === 4 ) {
 						callback();
 					} else {
-						// Create the active xhrs callbacks list if needed
-						// and attach the unload handler
-						if ( !xhrCallbacks ) {
-							xhrCallbacks = {};
-							xhrOnUnloadAbort();
+						handle = ++xhrId;
+						if ( xhrOnUnloadAbort ) {
+							// Create the active xhrs callbacks list if needed
+							// and attach the unload handler
+							if ( !xhrCallbacks ) {
+								xhrCallbacks = {};
+								jQuery( window ).unload( xhrOnUnloadAbort );
+							}
+							// Add to list of active xhrs callbacks
+							xhrCallbacks[ handle ] = callback;
 						}
-						// Add to list of active xhrs callbacks
-						handle = xhrId++;
-						xhr.onreadystatechange = xhrCallbacks[ handle ] = callback;
+						xhr.onreadystatechange = callback;
 					}
 				},