XMLHttpRequest Ajax Error

Question

I m using XMLHttpRequest in JavaScript  However  it gives me an error  and I don t know what my problem is  I have to parse an XML file and assign its contents to the webpage - here s my code    lt script    text javascript  gt       window onload   onPageLoad        var questionNum   0       function onPageLoad questionNum            var xmlhttp   new XMLHttpRequest            xmlhttp open  GET   quiz xml            try               xmlhttp send null      Here a xmlhttprequestexception number 101 is thrown            catch err                document getElementById  body   innerHTML      nXMLHttprequest error      err description     This prints  XMLHttprequest error  undefined  in the body                    xmlDoc   xmlhttp responseXML          parser   new DOMParser       This code is untested as it does not run this far         lt  script gt    My XML file is inside the same directory     lt question gt       lt query gt what is 2 2  lt  query gt       lt option gt 4 lt  option gt       lt option gt 5 lt  option gt       lt option gt 3 lt  option gt       lt answer gt 4 lt  answer gt   lt  question gt    Just for reference  I typically program in C  or Java  and I m running my website on Google Chrome

User · Answer

The problem is likely to lie with the line:

window.onload = onPageLoad();

By including the brackets you are saying onload should equal the return value of onPageLoad(). For example:

/*Example function*/
function onPageLoad()
{
    return "science";
}
/*Set on load*/
window.onload = onPageLoad()

If you print out the value of window.onload to the console it will be:

science

The solution is remove the brackets:

window.onload = onPageLoad;

So, you're using onPageLoad as a reference to the so-named function.

Finally, in order to get the response value you'll need a readystatechange listener for your XMLHttpRequest object, since it's asynchronous:

xmlDoc = xmlhttp.responseXML;
parser = new DOMParser(); // This code is untested as it doesn't run this far.

Here you add the listener:

xmlHttp.onreadystatechange = function() {
    if(this.readyState == 4) {
        // Do something
    }
}

User · Answer

So there might be a few things wrong here  First start by reading how to use XMLHttpRequest open   because there s a third optional parameter for specifying whether to make an asynchronous request  defaulting to true  That means you re making an asynchronous request and need to specify a callback function before you do the send    Here s an example from MDN  var oXHR   new XMLHttpRequest     oXHR open  quot GET quot    quot http   www mozilla org  quot   true    oXHR onreadystatechange   function  oEvent        if  oXHR readyState     4            if  oXHR status     200              console log oXHR responseText            else              console log  quot Error quot   oXHR statusText                       oXHR send null    Second  since you re getting a 101 error  you might use the wrong URL  So make sure that the URL you re making the request with is correct  Also  make sure that your server is capable of serving your quiz xml file  You ll probably have to debug by simplifying narrowing down where the problem is  So I d start by making an easy synchronous request so you don t have to worry about the callback function  So here s another example from MDN for making a synchronous request  var request   new XMLHttpRequest    request open  GET    file    home user file json   false    request send null    if  request status    0      console log request responseText    Also  if you re just starting out with Javascript  you could refer to MDN for Javascript API documentation examples tutorials

User · Answer

I see 2 possible problems   Problem 1   the XMLHTTPRequest object has not finished loading the data at the time you are trying to use it   Solution  assign a callback function to the objects  onreadystatechange  -event and handle the data in that function  xmlhttp onreadystatechange   callbackFunctionName    Once the state  has reached DONE  4   the response content is ready to be read   Problem 2   the XMLHTTPRequest object does not exist in all browsers  by that name    Solution   Either use a try-catch for creating the correct object for correct browser   ActiveXObject in IE  or use a framework  for example jQuery ajax-method  Note  if you decide to use jQuery ajax-method  you assign the callback-function with jqXHR done

[javascript] XMLHttpRequest (Ajax) Error

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