纯JS function getElementsByClass

/**
* rootNode          the root node to be checked on,
*                   for example 'document'
* classToSearch     className to be matched
* tagName           elements name to be searched between,
*                   '*' for all elements in 'rootNode'
* returns           an array contains all elements matched
*/
function getByClass(rootNode, classToSearch, tagName) {
var elementsToReturn = new Array();
var elementList = rootNode.getElementsByTagName(tagName);
var nLen = elementList.length;
var pattern = new RegExp("" + classToSearch + "");
for (var i = 0; i < nLen; i++) {
   if (pattern.test(elementList[i].className)) {
    elementsToReturn[elementsToReturn.length] = elementList[i];
   }
}
return elementsToReturn;
}