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Ajax's Onreadystatechange Return Uncaught Typeerror While Trying The Example In The Book

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getXMLHttpRequestObject returns nothing, which means it returns undefined. The ajax variable inside that function is not the same as the one outside. So I have to return variable ajax at the end of both functions.


window.onload = function(){

     'use strict';
     var ajax = getXMLHttpRequestObject();
     ajax.onreadystatechange = function(){
         if(ajax.readyState == 4){
             if((ajax.status >= 200 && ajax.status < 300) || (ajax.status == 304)){

                 document.getElementById('output').innerHTML = ajax.responseText;
                 document.getElementById('output').innerHTML = 'Error ' + ajax.statusText;
         document.getElementById('btn').onclick = function(){
         ajax.open('GET', 'resources/test.txt', true);

     //return var ajax
     **return ajax;**
 function getXMLHttpRequestObject(){
     var ajax = null;
         ajax = new XMLHttpRequest();
     }else if(window.ActiveXObject){
         ajax = new ActiveXObject('MSXML2.XMLHTTP.3.0');

     //return var ajax
     **return ajax**

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