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 · Accepted 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("GET", "http://www.mozilla.org/", true);

oXHR.onreadystatechange = function (oEvent) {
    if (oXHR.readyState === 4) {
        if (oXHR.status === 200) {
          console.log(oXHR.responseText)
        } else {
           console.log("Error", 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

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

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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