How do I load and parse a local XML file?

Load the file using an AJAX request from a local URL, then parse it using the DOMParser class.

//create a setupRealData function that I can call from somewhere in my app
StartAssistant.prototype.setupRealData = function() {
    var url = "content/content.xml";
    var request = new Ajax.Request(url, {
        method: 'get',
        evalJSON: 'false', //to enforce parsing JSON if there is JSON response
        onCreate: function(){'******* onCreate happened')},
        onLoading: function(){'******* onLoading happened')},
        onLoaded: function(){'******* onLoaded happened')},
        onSuccess: function(){'******* onComplete happened')},
        onComplete: this.gotResults.bind(this),
        onFailure: this.failure.bind(this)
//this function will by called when AJAX request above returns successfully
StartAssistant.prototype.gotResults = function(event){
    console.log("got the results");
    var xmlstring = event.responseText;		
    // parse into a DOM document object
    var xmlobject = (new DOMParser()).parseFromString(xmlstring, "text/xml");
    // Use xpath to get the parts of the XML I care about
    var nodes = document.evaluate("/content/section", xmlobject, null, XPathResult.ANY_TYPE, null);
    var section = nodes.iterateNext();
    while (section)
        console.log("section = " + section);

Learn More