Read and edit local xml-file in Javascript

I have a directory that contains

--index.html
--script.js
--file.xml

and I want to read the content of the file.xml as a string in order to replace a few words within the xml-file. (Later on I would like to send the edited xml-file as e-mail).

I managed to access to xml-file, but did not manage to save the whole content in a string and/or replace some known keywords within the xml.

This is where I got so far:

var oXHR = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');

function reportStatus2() {
  if (oXHR.readyState == 4)               // REQUEST COMPLETED.
     console.log(this.responseXML)        // This gets me the XML-document, but I want the complete xml content as string
}
oXHR.onreadystatechange = reportStatus2;    
oXHR.open("GET", "../file.xml", true);
oXHR.send();

I found so many posts on that topic here, but none was able to answer my question!
Any ideas?