Why XMLHttpRequest return null?

Question

This is a bit strange. I ran this script in different computer and got different result. In one computer, this code worked but didn't work in another. Here's my js code:

xmlDoc = loadXmlDoc();
printSubClassOf();
function loadXmlDoc(){
 var xmlDom = null;
 var xhttp = new XMLHttpRequest();
 xhttp.open("GET", "resto.owl", false);
 xhttp.send(null);
 xmlDom = xhttp.responseXML;
 return xmlDom;
}
function printSubClassOf(){
 a = xmlDoc.getElementsByTagName("SubClassOf"); 
 for(i = 0; i < a.length; i++){
 b = xmlDoc.getElementsByTagName("SubClassOf")[i].childNodes;
 for(j = 0; j < b.length; j++){ 
 c = xmlDoc.getElementsByTagName("SubClassOf")[i].childNodes[j];
 document.write(i + " " + j + " " + c.nodeName + "<br>");
 }
 }
}

And this is the XML I tried to parse.

<Ontology>
 <SubClassOf>
 <Class IRI="#Chef"/>
 <Class IRI="#Employee"/>
 </SubClassOf>
 <SubClassOf>
 <Class IRI="#Customer"/>
 <Class IRI="#People"/>
 </SubClassOf>
</Ontology>

I used Chrome Developer Tools to see what was going on. There was a XHR request with 200 status code but still got this error.

a = xmlDoc.getElementsByTagName("SubClassOf");
Uncaught TypeError: Cannot call method 'getElementsByTagName' of null

I think printSubClassOf() finished before loadXmlDoc() returns the XML. What do you think? And how to solve this problem? FYI: I ran this on localhost.

• bfavaretto: I tried the code you posted, and it worked. Are you trying that exact same code, or something different?

Answer 1

Your xml is invalid. There needs to be a single root node to be valid xml. responseXML will only be populated if the response is valid xml.

• Musa: @shankshera is the content type of the file set to text/xml
• bfavaretto: I believe you're right, it must be the content type. I tried the OP code using a .xml file, apache served it with the xml content type, and it worked. Renamed the file to .owl, and got the exact same error as the OP.
• fi0rini: Thanks, this helped me... My responseText and response variables were populating after request but ResponseXML was null. After editing the XML, my requestXML variable populated as I expected.