A simple example of overloading XMLHttpRequest to always send a custom header

alwaysSendHeader.js

// get real XMLHttpRequest object
var RealXMLHttpRequest = window.XMLHttpRequest;

// create the overloaded XMLHttpObject
window.XMLHttpRequest = function(){
    // just in case someone is still using IE6, use a fallback
    var ajax = !!RealXMLHttpRequest ? new RealXMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP"),
        realOpen = ajax.open;

    // steal the 'open' function, since the request has o be open
    // in order to send headers
    ajax.open = function(method, url){
        realOpen.apply(ajax, arguments);

        // note: here, yu have access to the method (GET, POST, etc.), as well
        // as the url for the request... so 'if' to your content

        // automatically send the header once open
        ajax.setRequestHeader('X-Catdad-Custom', 'meow');
    };

    return ajax;
};