AJAX timing interceptor: this class intercepts all AJAX calls and records the time taken for the HTTP request to complete. These timings are posted back to the server in batches, if there are any to send, about every two seconds. Tested in Firefox, Chrome

intercept.js

(function(XHR) {
    "use strict";
    
    var stats = [];
    var timeoutId = null;
    
    var open = XHR.prototype.open;
    var send = XHR.prototype.send;
    
    XHR.prototype.open = function(method, url, async, user, pass) {
        this._url = url;
        open.call(this, method, url, async, user, pass);
    };
    
    XHR.prototype.send = function(data) {
        var self = this;
        var start;
        var oldOnReadyStateChange;
        var url = this._url;
        
        function onReadyStateChange() {
            if(self.readyState == 4 /* complete */) {
                var time = new Date() - start;                
                stats.push({
                    url: url,
                    duration: time                    
                });
                
                if(!timeoutId) {
                    timeoutId = window.setTimeout(function() {
                        var xhr = new XHR();
                        xhr.noIntercept = true;
                        xhr.open("POST", "/clientAjaxStats", true);
                        xhr.setRequestHeader("Content-type","application/json");
                        xhr.send(JSON.stringify({ stats: stats } ));                        
                        
                        timeoutId = null;
                        stats = []; 
                    }, 2000);
                }                
            }
            
            if(oldOnReadyStateChange) {
                oldOnReadyStateChange();
            }
        }
        
        if(!this.noIntercept) {
            start = new Date();
            
            if(this.addEventListener) {
                this.addEventListener("readystatechange", onReadyStateChange, false);
            } else {
                oldOnReadyStateChange = this.onreadystatechange; 
                this.onreadystatechange = onReadyStateChange;
            }
        }
        
        send.call(this, data);
    }
})(XMLHttpRequest);