Why is my XML returned from readystatechange empty [duplicate]

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• How to return the response from an Ajax call? 6 answers

xmlresponse.onreadystatechange=function(){
if (xmlresponse.readyState==4 && xmlresponse.status==200){
        document.xml_fragment = xmlresponse.responseXML.documentElement.childNodes;
        };
};
var streamlist = document.xml_fragment.getElementsByTagName("stream");

Why is document.xml_fragment in the last line not defined?