document.getElementByClassName - Cross Browser Fix

 
var getElementsByClassName = function (className, tag, elm){
 if (document.getElementsByClassName) {
  getElementsByClassName = function (className, tag, elm) {
   elm = elm || document;
   var elements = elm.getElementsByClassName(className),
    nodeName = (tag)? new RegExp("\\b" + tag + "\\b", "i") : null,
    returnElements = [],
    current;
   for(var i=0, il=elements.length; i<il; i+=1){
    current = elements[i];
    if(!nodeName || nodeName.test(current.nodeName)) {
     returnElements.push(current);
    }
   }
   return returnElements;
  };
 }
 else if (document.evaluate) {
  getElementsByClassName = function (className, tag, elm) {
   tag = tag || "*";
   elm = elm || document;
   var classes = className.split(" "),
    classesToCheck = "",
    xhtmlNamespace = "http://www.w3.org/1999/xhtml",
    namespaceResolver = (document.documentElement.namespaceURI === xhtmlNamespace)? xhtmlNamespace : null,
    returnElements = [],
    elements,
    node;
   for(var j=0, jl=classes.length; j<jl; j+=1){
    classesToCheck += "[contains(concat(' ', @class, ' '), ' " + classes[j] + " ')]";
   }
   try {
    elements = document.evaluate(".//" + tag + classesToCheck, elm, namespaceResolver, 0, null);
   }
   catch (e) {
    elements = document.evaluate(".//" + tag + classesToCheck, elm, null, 0, null);
   }
   while ((node = elements.iterateNext())) {
    returnElements.push(node);
   }
   return returnElements;
  };
 }
 else {
  getElementsByClassName = function (className, tag, elm) {
   tag = tag || "*";
   elm = elm || document;
   var classes = className.split(" "),
    classesToCheck = [],
    elements = (tag === "*" && elm.all)? elm.all : elm.getElementsByTagName(tag),
    current,
    returnElements = [],
    match;
   for(var k=0, kl=classes.length; k<kl; k+=1){
    classesToCheck.push(new RegExp("(^|\\s)" + classes[k] + "(\\s|$)"));
   }
   for(var l=0, ll=elements.length; l<ll; l+=1){
    current = elements[l];
    match = false;
    for(var m=0, ml=classesToCheck.length; m<ml; m+=1){
     match = classesToCheck[m].test(current.className);
     if (!match) {
      break;
     }
    }
    if (match) {
     returnElements.push(current);
    }
   }
   return returnElements;
  };
 }
 return getElementsByClassName(className, tag, elm);
};

Note:

We have created a global function called getElementsByClassName, not a method on the document object.

We need to call getElementsByClassName or window.getElementsByClassName, not document.getElementsByClassName.

Reference: http://robertnyman.com/2008/05/27/the-ultimate-getelementsbyclassname-anno-2008/