Javascript Getting specific element (of parent) by name

Using getElementsByTagName() and getElementsByName() will return a NodeList, you need to get the first element of the list like this:

var parent = document.getElementsByTagName('mysection')[0];
var myfield = parent.getElementsByName("myfield")[0];

Edit

You were correct, getElementsByName is not valid for an element. I am unsure how to localize the functionality of it as you are trying to do. It seems that it will only work for document. You may have to write your own implementation of getElementsByName if you want to use it in a localized scope.

Second Edit

To be nice, I made that implementation for you :D Here it is in all its "glory".

Element.prototype.getElementsByName = function (arg) {
    var returnList = [];
    (function BuildReturn(startPoint) {
        for (var child in startPoint) {
            if (startPoint[child].nodeType != 1) continue; //not an element
            if (startPoint[child].getAttribute("name") == arg) returnList.push(startPoint[child]);
            if (startPoint[child].childNodes.length > 0) {
                BuildReturn(startPoint[child].childNodes);
            }
        }
    })(this.childNodes);
    return returnList;
};
var parent = document.getElementsByTagName('mysection')[0];
var myfield = parent.getElementsByName("myfield")[0];

Small fix

I was incorrectly passing the element and not its children into the recursion. The code above has been edited with the proper argument passed now. See working fiddle: http://jsfiddle.net/js6NP/5/

I actually found a much more simple way to handle this:

document.querySelectorAll('mysection [name="myfield"]');

Here you can see an example where it only modifies the field inside the section specified: http://jsfiddle.net/fluidbyte/kph6H/

qSA supports modern browsers and is compatible down to IE8, Here's a polyfill to support back to IE7: https://gist.github.com/2724353