getElementsByClassName.js

/**
 * [getElementsByClassName]
 * @param  {[String]} searchClass [searchClass]
 * @param  {[Object]} node        [node document]
 * @param  {[String]} tag         [html tag]
 * @return {[Array]}              [result]
 *
 * @from http://www.cnblogs.com/rubylouvre/archive/2009/07/24/1529640.html
 */
var getElementsByClassName = function(searchClass, node, tag) {
    if (document.getElementsByClassName) {
        var nodes = (node || document).getElementsByClassName(searchClass),
            result = [];
        for (var i = 0; node = nodes[i++];) {
            if (tag !== "*" && node.tagName === tag.toUpperCase()) {
                result.push(node)
            }
        }
        return result
    } else {
        node = node || document;
        tag = tag || "*";
        var classes = searchClass.split(" "),
            elements = (tag === "*" && node.all) ? node.all : node.getElementsByTagName(tag),
            patterns = [],
            current,
            match;
        var i = classes.length;
        while (--i >= 0) {
            patterns.push(new RegExp("(^|\\s)" + classes[i] + "(\\s|$)"));
        }
        var j = elements.length;
        while (--j >= 0) {
            current = elements[j];
            match = false;
            for (var k = 0, kl = patterns.length; k < kl; k++) {
                match = patterns[k].test(current.className);
                if (!match) break;
            }
            if (match) result.push(current);
        }
        return result;
    }
}

getElementsByClassName2.js

function getElementsByClassName(node, classname) {
    if (node.getElementsByClassName) { // use native implementation if available
        return node.getElementsByClassName(classname);
    } else {
        return (function(searchClass, node) {
            if (node == null)
                node = document;
            var classElements = [],
                els = node.getElementsByTagName("*"),
                elsLen = els.length,
                pattern = new RegExp("(^|\\s)" + searchClass + "(\\s|$)"),
                i, j;

            for (i = 0, j = 0; i < elsLen; i++) {
                if (pattern.test(els[i].className)) {
                    classElements[j] = els[i];
                    j++;
                }
            }
            return classElements;
        })(classname, node);
    }
}