The missing function "getElementsByClassName()"

getElementsByClassName.js

function getElementsByClassName(class_name) {
    var all_obj, ret_obj=new Array(), j=0, teststr;
    
    if(document.all) {
        all_obj = document.all;
    }
    else if(document.getElementsByTagName && !document.all){
        all_obj = document.getElementsByTagName("*");
    }
    
    for(var i = 0; i < all_obj.length; i++) {
        
        if(all_obj[i].className.indexOf(class_name) != -1) {
            teststr = "," + all_obj[i].className.split(" ").join(",") + ",";
            
            if(teststr.indexOf("," + class_name + ",") !=-1) {
                ret_obj[j] = all_obj[i];
                j++;
            }
        }
    }
    return ret_obj;
}