var i égale à quoi une fois que j = 3 dans une boucle

var xmlDoc=null;
if (window.XMLHttpRequest)
  {
  xhttp=new XMLHttpRequest();
  }
else
  {
  xhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xhttp.open("GET","test.xml",false);
xhttp.send("");
xmlDoc=xhttp.responseXML;
var x=xmlDoc.getElementsByTagName("post");

<post no="1" titre="Superbe video" date="12 mars 2011"  categorie="video" id="xeac9f"/>

var iCount = 0;
var iVal;
for(var i = 0; i < x.length; i++){
  iVal= x[i].getAttribute("categorie");
  if(iVal == "video"){ iCount++;}
}

var iCount = 0;
var iVal;

for(var i = 0; i < x.length; i++){
  iVal= x[i].getAttribute("categorie");
  
if(iVal == "video"){ iCount++;}

if(iCount == 3){ alert(i);}

}

var iCount = 0;
var iVal;

for(var i = 0; i < x.length; i++){
  iVal= x[i].getAttribute("categorie");
  
if(iVal == "video"){ iCount++;}

if(iCount == 3){ var reponse = i}

}

alert(reponse);