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);

I'm kind of new to javascript
Could anyone help me to understand how to use this?

@Rkundan, @llloyola
Just load the file / execute in console. Every time an AJAX call is made, the code will trigger. Specifically in line 34, he's setting to send AJAX information to own server URL "/clientAjaxStats". This should be modified to your needs

I'm kind of new to javascript
Could anyone help me to understand how to use this?

No, please you explain why you'd need this being new to JS :) really, just curious!

@suprememoocow , should this work in all cases? Or it is possible that some code uses a different method to fetch data, and therefore bypass this? I've been experimenting with this, and find that it only works in some cases, but I have not been able to discover why.

Thank you for such a nice code. I did couple changes to match my needs.

(function (XHR) {
	"use strict";

	var open = XHR.prototype.open;
	var send = XHR.prototype.send;
	var setRequestHeader = XHR.prototype.setRequestHeader;

	var interceptBefore = [];
	var interceptAfter = [];
	var requestHeaders = {};

	XHR.prototype.setRequestHeader = function (header, value) {
		requestHeaders[header] = value;
		setRequestHeader.call(this, header, value);
	};

	XHR.prototype.open = function (method, url, async, user, pass) {
		this._url = url;
		this._method = method;
		open.call(this, method, url, async, user, pass);
	};

	XHR.prototype.send = function (data) {
		var self = this;
		var primeOnReadyStateChange = this.onReadyStateChange;

		for (var i = 0; i < interceptBefore.length; i += 1) {
			interceptBefore[i].apply(null, [
				this,
				decodeURIComponent(data),
				decodeURIComponent(this._url),
				this._method.toLocaleLowerCase(),
				requestHeaders,
			]);
		}

		function onReadyStateChange() {
			if (self.readyState == 4 /* complete */) {
				for (var i = 0; i < interceptAfter.length; i += 1) {
					interceptAfter[i](self);
				}
			}

			if (primeOnReadyStateChange) {
				primeOnReadyStateChange();
			}
		}

		if (this.addEventListener) {
			this.addEventListener(
				"readystatechange",
				onReadyStateChange,
				false
			);
		} else {
			this.onreadystatechange = onReadyStateChange;
		}

		send.call(this, data);
	};

	XHR.interceptBefore = function (callback) {
		interceptBefore.push(callback);
	};

	XHR.interceptAfter = function (callback) {
		interceptAfter.push(callback);
	};
})(XMLHttpRequest);

The example:

XMLHttpRequest.interceptBefore((XHR, data, url, method, requestHeaders) => {
	console.log("BEFORE: ");
	console.log(XHR);
	console.log(method);
	console.log(data);
	console.log(url);
	console.log(requestHeaders);
});

XMLHttpRequest.interceptAfter((XHR) => {
	console.log(
		"AFTER: ",
		XHR,
		XHR.getAllResponseHeaders()
			.split("\r\n")
			.filter(Boolean)
			.reduce(
				(acc, x) => ({ ...acc, [x.split(": ")[0]]: x.split(": ")[1] }),
				{}
			),
		XHR.responseURL,
		XHR.responseText
	);
});