jQuery.ajaxSettings.xhr = function() {
	try {
		return new XMLHttpRequest();
	} catch( e ) {}
};

var xhrSupported = jQuery.ajaxSettings.xhr(),
	xhrSuccessStatus = {
		// file protocol always yields status code 0, assume 200
		0: 200,
		// Support: IE9
		// #1450: sometimes IE returns 1223 when it should be 204
		1223: 204
	},
	// Support: IE9
	// We need to keep track of outbound xhr and abort them manually
	// because IE is not smart enough to do it all by itself
	xhrId = 0,
	xhrCallbacks = {};

if ( window.ActiveXObject ) {
	jQuery( window ).on( "unload", function() {
		for( var key in xhrCallbacks ) {
			xhrCallbacks[ key ]();
		}
		xhrCallbacks = undefined;
	});
}

jQuery.support.cors = !!xhrSupported && ( "withCredentials" in xhrSupported );
jQuery.support.ajax = xhrSupported = !!xhrSupported;

jQuery.ajaxTransport(function( options ) {
	var callback;
	// Cross domain only allowed if supported through XMLHttpRequest
	if ( jQuery.support.cors || xhrSupported && !options.crossDomain ) {
		return {
			send: function( headers, complete ) {
				var i, id,
					xhr = options.xhr();
				xhr.open( options.type, options.url, options.async, options.username, options.password );
				// Apply custom fields if provided
				if ( options.xhrFields ) {
					for ( i in options.xhrFields ) {
						xhr[ i ] = options.xhrFields[ i ];
					}
				}
				// Override mime type if needed
				if ( options.mimeType && xhr.overrideMimeType ) {
					xhr.overrideMimeType( options.mimeType );
				}
				// X-Requested-With header
				// For cross-domain requests, seeing as conditions for a preflight are
				// akin to a jigsaw puzzle, we simply never set it to be sure.
				// (it can always be set on a per-request basis or even using ajaxSetup)
				// For same-domain requests, won't change header if already provided.
				if ( !options.crossDomain && !headers["X-Requested-With"] ) {
					headers["X-Requested-With"] = "XMLHttpRequest";
				}
				// Set headers
				for ( i in headers ) {
					xhr.setRequestHeader( i, headers[ i ] );
				}
				// Callback
				callback = function( type ) {
					return function() {
						if ( callback ) {
							delete xhrCallbacks[ id ];
							callback = xhr.onload = xhr.onerror = null;
							if ( type === "abort" ) {
								xhr.abort();
							} else if ( type === "error" ) {
								complete(
									// file protocol always yields status 0, assume 404
									xhr.status || 404,
									xhr.statusText
								);
							} else {
								complete(
									xhrSuccessStatus[ xhr.status ] || xhr.status,
									xhr.statusText,
									// Support: IE9
									// #11426: When requesting binary data, IE9 will throw an exception
									// on any attempt to access responseText
									typeof xhr.responseText === "string" ? {
										text: xhr.responseText
									} : undefined,
									xhr.getAllResponseHeaders()
								);
							}
						}
					};
				};
				// Listen to events
				xhr.onload = callback();
				xhr.onerror = callback("error");
				// Create the abort callback
				callback = xhrCallbacks[( id = xhrId++ )] = callback("abort");
				// Do send the request
				// This may raise an exception which is actually
				// handled in jQuery.ajax (so no try/catch here)
				xhr.send( options.hasContent && options.data || null );
			},
			abort: function() {
				if ( callback ) {
					callback();
				}
			}
		};
	}
});

src/ajax/xhr.js

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