getElementsByClassName polyfill

function.js

/*jshint forin:true, noarg:true, noempty:true, eqeqeq:true, bitwise:true, strict:true, undef:true, curly:true, browser:true, indent:2, maxerr:50 */
(function (document) {
  "use strict";
  function getElementsByClassName(match, tag) {
    if (document.getElementsByClassName) {
      return document.getElementsByClassName(match);
    }
    var result = [],
      elements = document.getElementsByTagName(tag || '*'),
      i, elem; 
    match = " " + match + " ";
    for (i = 0; i < elements.length; i++) { 
      elem = elements[i];
      if ((" " + (elem.className || elem.getAttribute("class")) + " ").indexOf(match) > -1) {
        result.push(elem);
      }
    }
    return result; 
  }
}(document));

polyfill.js

/*jshint forin:true, noarg:true, noempty:true, eqeqeq:true, bitwise:true, strict:true, undef:true, curly:true, browser:true, indent:2, maxerr:50 */
(function (document) {
  "use strict";
  if (document.getElementsByClassName) {
    document.getElementsByClassName = function (match) { 
      var result = [],
        elements = document.getElementsByTagName('*'),
        i, elem;
      match = " " + match + " ";
      for (i = 0; i < elements.length; i++) { 
        elem = elements[i];
        if ((" " + (elem.className || elem.getAttribute("class")) + " ").indexOf(match) > -1) {
          result.push(elem);
        }
      }
      return result; 
    };
  }
}(document));

You’ll probably want to replace /[\t\n\r]/g with /[\t\n\f\r]/g as per http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#space-character:

The space characters, for the purposes of this specification, are U+0020 SPACE, U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), U+000C FORM FEED (FF), and U+000D CARRIAGE RETURN (CR).

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Also:

if ((" " + (elem.className || elem.getAttribute("class")) + " ").indexOf(match) > -1) {

I think you mean:

if ((" " + (elem.className || elem.getAttribute("class")) + " ").indexOf(" " + match + " ") > -1) {

Else, document.getElementsByClassName('oba') would match elements with e.g. class="foobarbaz".

Original piece of code was taken from sizzle. I don't think we need so complex logic for this issue, so i simplified it.
In the second thought I understand it could be not polyfill but just local function.

The updated code still has the problem I described, e.g. document.getElementsByClassName('oba') would match elements with class="foobarbaz".

"" + ("this is the value of the class attribute").indexOf("ass") > -1; // true

now i get it why there were those spaces

fixed

match = " " + match + " ";

In the polifill.js
Shouldn't the third line be
if (!document.getElementsByClassName) {
instead of
if (document.getElementsByClassName) {