Q:

Is it possible to send an AJAX request on load?

Hello I have two dependants select box, the second one is popularited after onchange event.
The first one is loaded with a selected value (selected=selected), what I'm asking, it is possible to send the requested while the page is loading, ie as I have the the selected option, just send the request.
Javascript
function getXMLHTTP() {
        var xmlhttp=false;  
        try{
            xmlhttp=new XMLHttpRequest();
        }
        catch(e)    {       
            try{            
                xmlhttp= new ActiveXObject("Microsoft.XMLHTTP");
            }
            catch(e){
                try{
                xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
                }
                catch(e1){
                    xmlhttp=false;
                }
            }
        }

        return xmlhttp;
    }

    function getSubCat(catId,incat) {   
        var strURL="../Includes/subcatAds.php?SubCat="+catId+"&incat="+incat;
        var req = getXMLHTTP();

        if (req) {

            req.onreadystatechange = function() {
                if (req.readyState == 4) {
                    // only if "OK"
                    if (req.status == 200) {                        
                        document.getElementById('subcat').innerHTML=req.responseText;                       
                    } else {
                        alert("There was a problem while using XMLHTTP:\n" + req.statusText);
                    }
                }               
            }           
            req.open("GET", strURL, true);
            req.send(null);
        }       
    }

The PHP will be provided if needed

A:

You really need to use jQuery and replace all of the above with:
function getSubCat(catId, incat)
  { 
    $('#subcat').load("../Includes/subcatAds.php?SubCat="+catId+"&incat="+incat);
  }

// Run on load:
$( function(){ 
               getSubCat(4, 5);
   });

It will do the same thing.  It's set up to run on load, and you don't have to worry about cross browser compatibility.
You will find yourself using jQuery selectors all the time and your code will be very  lightweight.  If you link the jQuery library to Google servers people will not even have to download it.  Most people have it in cache already.

