Comment faire une requête AJAX pour publier des données JSON et traiter la réponse

OK, Voici comment faire les deux GET et POST demandes de vanille dans le javascript, j'.e n tiers des bibliothèques comme jQuery, y compris la façon de définir des en-têtes de requête:

//Just to namespace our functions and avoid collisions
var _SU3 = _SU3 ? _SU3 : new Object();
//Does a get request
//url: the url to GET
//callback: the function to call on server response. The callback function takes a
//single arg, the response text.
_SU3.ajax = function(url, callback){
var ajaxRequest = _SU3.getAjaxRequest(callback);
ajaxRequest.open("GET", url, true);
ajaxRequest.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
ajaxRequest.send(null); 
};
//Does a post request
//callback: the function to call on server response. The callback function takes a
//single arg, the response text.
//url: the url to post to
//data: the json obj to post
_SU3.postAjax = function(url, callback, data) {
var ajaxRequest = _SU3.getAjaxRequest(callback);
ajaxRequest.open("POST", url, true);
ajaxRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
ajaxRequest.setRequestHeader("Connection", "close");
ajaxRequest.send("data=" + encodeURIComponent(data));    
};
//Returns an AJAX request obj
_SU3.getAjaxRequest = function(callback) {
var ajaxRequest;
try {
ajaxRequest = new XMLHttpRequest();
} catch (e) {
try { 
ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e){
return null;
}
}
}
ajaxRequest.onreadystatechange = function() {
if (ajaxRequest.readyState == 4) {      
//Prob want to do some error or response checking, but for 
//this example just pass the responseText to our callback function
callback(ajaxRequest.responseText);
}
};
return ajaxRequest;
}; 

L'utiliser comme ceci:

function processResponse(responseText) {
//Response text is a json:
var obj = JSON.parse(responseText)       //won't work all browsers, there are alternatives
//Do something with obj
....
}
var jsonToPost = ....     //whatever your json is
var url = ...   //the URL to post to
_SU3.postAjax(url, processResponse, jsonToPost);