XMLHttpRequest() kdyz posilam data (post) tak neuklada

function Lra(j){u=0;for(r in j){u=(u==0)?j[r]:u+"&"+j[r];}r=new XMLHttpRequest();r.onreadystatechange=function(){if(r.readyState==4){
if(r.status==200){window.alert(r.responseText);}}};r.open('post','url. php',true);
r.setRequestHeader("Content-type","application/x-www-form-urlencoded") ;r.send(u);}
vsechno mi funguje, i ulozi poslana data ale proste mi to nikdy nevrati zadny text(r.responseText).prestoze ma vratit.Nevite cim to je?

tak je to timhle:
r.setRequestHeader("Content-type","application/x-www-form-urlencoded") ;
ale co kdyz to potrebuju, a presto potrebuju aby mi to vratilo tech pres responseText??nikde to nemuzu najit