GetHTTPObject Function Not Valid On Web Browser

Got this snippet of code, but getHTTPObject(); is not valid. What .js library need to get this working?
<script src="json2.js"></script>
var request;
function runAjax(JSONstring){
// function returns "AJAX" object, depending on web browser
// this is not native JS function!
request = getHTTPObject();
request.onreadystatechange = sendData;
request.open("GET", "parser.php?json="+JSONstring, true);
request.send(null);
}
// function is executed when var request state changes
function sendData(){
// if request object received response
if(request.readyState == 4)
// parser.php response
var JSONtext = request.responseText;
// convert received string to JavaScript object
var JSONobject = JSON.parse(JSONtext);
// notice how variables are used
var msg = "Number of errors: "+JSONobject.errorsNum+"
- "+JSONobject.error[0]+"
- "+JSONobject.error[1];
alert(msg);
}}