javascript - Why Chrome sometimes makes 2 AJAX requests instead of 1? -

i have encountered strange behaviour of current version of google chrome (44.0.2403.157) on os x.

when ajax (xhr) request method chrome sometimes makes 1 request , - 2 (!).

my code:

var jsurl = 'http://www.some-domain.com/xy.js';  var ajax = new xmlhttprequest();   ajax.withcredentials = true;   ajax.open( 'get', jsurl, true ); // async, because sync cannot send cookies   ajax.onreadystatechange = function () {       var script = ajax.response || ajax.responsetext;       if (ajax.readystate === 4) {           switch( ajax.status) {               case 200:               case 304:                 eval.apply( window, [script] );                 // no break on purpose here               default:                 somefunction();           }       }   };   ajax.send(null); 

a screenshot of developer console when 2 requests being made:

server log confirms second request being made - it's not in console.

the headers of second request practically same first 1 - difference value of cookie being changed 3rd request on screen above.

note current version of firefox (40.0.3) on os x doesn't show behaviour - 1 request made here, every time (tested both watching request using firebug (with bfcache requests shown) , watching server logs).

for while thought using send() vs send(null) makes difference - when using send() 1 request being made - after more tests see strangeness present both syntaxes.

can give me hint happening here?