javascript - 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.

Answer 1

您的 xml 无效。需要有一个根节点才能成为有效的 xml。只有当响应是有效的 xml 时才会填充 responseXML。