Toggle Class Prototype Function - magento

Question

I'm trying to iterate over an array of elements, specified by class name. Then I want to use a function to toggle the class and change some text.
This does NOT work:
$$('.btn').forEach( setButtonLoadingStateOn, this );
setButtonLoadingStateOn = function( btn ) {
btn.toggleClassName('loading');
btn.disable();
btn.select('span span')[0].update( "please wait..." );
}
This does NOT work:
$$('.btn').each( function(btn) {
btn.addClassName('loading');
btn.disable();
btn.select('span span')[0].innerHTML = "please wait...";
});
This also does NOT work:
setButtonLoadingState( '.btn', 'start' );
setButtonLoadingState = function( btnClass, loadState ) {
btnElem = $$( btnClass );
btnElem.each(function( el ){
if ( loadState == 'start' ) {
el.addClassName( 'loading' );
el.disable();
el.select('span span')[0].innerHTML = "please wait...";
} else {
el.removeClassName( 'loading' );
el.enable();
el.select('span span')[0].innerHTML = "buy now";
}
});
}
If I console.log() the element, I get the object (or array) I'm expecting, so I don't know what's wrong.
I've also tried several SO answers, including this one: Add or remove class with prototype
In case it matters, the platform is Magento CE 1.8.0.2. How can I update these button elements using Prototype? Thanks.
~ edit ~
There are several HTML elements on the page. They all look like this:
<button type="button" title="buy now" class="button btn" onclick="productAddToCart(this)">
<span><span>Quick Buy</span></span>
</button>

Answer 1

Thanks so much to all who contributed. The AJAX request was returning a hyphenated key in the JSON response:
{"msg-html":"blah blah blah"}
Reading this from the response object as follows was the problem:
var ajaxResult = transport.responseText.evalJSON();
var ajaxMsg = ajaxResult.msg-html;
Hyphens are not allowed in JSON keys, instead the object needs to be accessed as follows:
var ajaxMsg = ajaxResult["msg-html"];
... or remove the hyphens! Thanks again to all.
Full credit to this dude/dudette: http://developer.appcelerator.com/question/120227/parsing-json-with-hyphenated-key-names