Use charAt instead of string indexing for oldIE's sake. Fixes #14356.

src/ajax.js

@@ -78,7 +78,7 @@ function addToPrefiltersOrTransports( structure ) {
 			// For each dataType in the dataTypeExpression
 			while ( (dataType = dataTypes[i++]) ) {
 				// Prepend if requested
-				if ( dataType[0] === "+" ) {
+				if ( dataType.charAt( 0 ) === "+" ) {
 					dataType = dataType.slice( 1 ) || "*";
 					(structure[ dataType ] = structure[ dataType ] || []).unshift( func );
 

test/unit/ajax.js

@@ -1568,9 +1568,10 @@ module( "ajax", {
 //----------- jQuery.ajaxPrefilter()
 
 	ajaxTest( "jQuery.ajaxPrefilter() - abort", 1, {
+		dataType: "prefix",
 		setup: function() {
 			// Ensure prefix does not throw an error
-			jQuery.ajaxPrefilter("+prefix *", function( options, _, jqXHR ) {
+			jQuery.ajaxPrefilter("+prefix", function( options, _, jqXHR ) {
 				if ( options.abortInPrefilter ) {
 					jqXHR.abort();
 				}