Ajax Hook.js - A simple Ajax Callback Interceptor

ajaxhook.js

/*
	AjaxHook.js - A simple utility library for intercepting Ajax calls.

	This may be useful to inject simple interceptors to analyze pr capture Ajax traffic and may be even useful for automation tests to determine when an Ajax response returns.

	To use :
	1>Create a new AjaxHook object
	var hook = new AjaxHook();

	2>Install a hook
	hook.installHook([URL string / RegEx], function callback(xhr){.......});

	where xhr is the XMLHttpRequest Object

	URL String/RegEx is an expression that matches one or more URLs. If a URL String is given, the http/https prefix may be omitted. Also, it will match all URLs having the given string as the start of the URL.
	For example: "www.mywebsite.co" in the URL string will match https://wwww.mywebsite.co.in and also http://wwww.mywebsite.com/something


	3>To remove a hook:
	hook.removeHook([URL string/RegEx]);

	4>To remove all hooks:
	hook.removeAllHooks();


	One or more hooks may be chained together using multiple instances of AjaxHook. Both instances may link a different interceptor callback to the same URL

	NOTE: 
	1>The interceptor callback fires after the application defined onreadystatechanged event fires
*/

function AjaxHook(){

	var hooks = {};
	var initTime = new Date().getTime();
	var self = this;

	var trim = function(strdata){
		strdata.replace(/\s+/,'');
	};

	var oldOpen = XMLHttpRequest.prototype.open;
	var oldSend = XMLHttpRequest.prototype.send;

	var convertStrToRegExp = function(str){
		var targetExpr = str.substring(1,str.length-1);
		return new RegExp(targetExpr);
	};

	var getHookCallbackForUrl = function(url){
		var iter = 0;
		var urlPats = Object.keys(hooks);
		for (iter=0;iter<urlPats.length;iter++){
			if (convertStrToRegExp(urlPats[iter]).test(url)){
				return hooks[urlPats[iter]];
			}
		};

	};

	var adaptAsRegEx = function(urlPat){
		return new RegExp("^(http(s)?:\/\/)?" + urlPat + "(.)*$"); //For impicit regexp expressions
	};


	XMLHttpRequest.prototype.open = function(){
		var currentUrl = arguments[1];
		var callback = getHookCallbackForUrl(currentUrl);
		if (callback){
			this[initTime] = {};
			this[initTime].callback = callback;
		}else if (this[initTime]){
			this.onreadystatechange = this[initTime].orgCallback;
			delete this[initTime];
		}
		oldOpen.apply(this, arguments);
	};


	XMLHttpRequest.prototype.send = function(){
		var xhrObj = this;
		if (this[initTime] && this[initTime].callback){ 
			this[initTime].orgCallback = this.onreadystatechange;
			var handler = function(){
				if (xhrObj[initTime].orgCallback){
					xhrObj[initTime].orgCallback.apply(xhrObj,Array.prototype.slice.apply(arguments));
				} 
				xhrObj[initTime].callback.call(xhrObj, xhrObj);
			};
			this.onreadystatechange = handler;
		}	
		oldSend.apply(this, arguments);	

	};


	this.installHook = function(url, callback){
		var targetUrl = (url instanceof RegExp) ? url : adaptAsRegEx(url);
		hooks[targetUrl] = callback;
	};

	this.removeHook = function(url){
		var targetUrl = (url instanceof RegExp) ? url : adaptAsRegEx(url);
		delete hooks[targetUrl];
	};

	this.removeAllHooks = function(){
		hooks = {};
	};
};

Notes:

1. For every hook a URL String or RegEx may be provided. If URL string is provided, then internally its treated as a URL pattern starting with a "http://" or "https://" followed by the string and having one or more characters following the given string.
   For Example: "www.google" will match "http://www.google.com", "https://www.google.com" and even "http://www.google.co.in"
2. More than one hook can be installed for the same URL. For the same, one may create multiple instances of AjaxHook. Each instance may be attached to the same URL