We developed a web server that sits on IIS, runs pure javascript (through SpiderMonkey) and uses e4x for templating. I implemented Array.prototype.mapXml and it has proven to be one of the most valuable methods in our arsenal. It allows us to write code like this:

function renderPeople() { 
    var people = getArrayOfPeopleSomehow(); 
    return <div class="people"> 
        {people.mapXML(function(person) <div class="person">{person.getOtherPeopleStuff()}</div>} 
        </div>; 
}

It is easy to implement and hard to live without once you've used it.