Cross-browser js function to retrieve elements by classname from dom. Based on prototype js code

getElementsByClassName.js

var getElementsByXPath = function(xpath, parentElement) {
    var xpathResult = document.evaluate(xpath, parentElement, null, XPathResult.ANY_TYPE, null);
	var results = [];
	var element = xpathResult.iterateNext();
	while(element) {
		results.push(element);
		element = xpathResult.iterateNext();
	}
	return results;
}

function hasClassName(element, className) {
    return (' ' + element.className + ' ').indexOf(' ' + className + ' ') > -1;
}

var getElementsByClassName = function(className, parentElement) {
    if (parentElement.getElementsByClassName) {
        return parentElement.getElementsByClassName(className);
    } else if (!!document.evaluate) { //detect xpath like prototype does
        var q = ".//*[contains(concat(' ', @class, ' '), ' " + className + " ')]";
        return getElementsByXPath(q, parentElement);
    } else {
        var children = parentElement.getElementsByTagName('*');
        var elements = [], child;
        for (var i = 0, length = children.length; i < length; i++) {
            child = children[i];
            if (hasClassName(child, className)) {
                elements.push(child);
            }
        }
        return elements;
    }
};

Some tests http://ejohn.org/blog/getelementsbyclassname-speed-comparison/