Upgraded from 2.5 to 3.5 : Joomla.JMultiSelect, Class is not defined

Question

I've upgraded an old site from 2.5 to 3.5 and have managed to fix everything except this error :

Uncaught ReferenceError: Class is not defined @ multiselect.js:12 
Uncaught TypeError: Joomla.JMultiSelect is not a function @ index.php?option=com_content:80

It seems like Joomla admin is not loading something correctly. I've uploaded joomla installation and overwritten everything with latest joomla files in case something didn't copy during auto upgrade but it didn't help.

I have tried cleaning my cookies and cache as well but it didn't help.

Here is the piece of code in question :

Joomla = Joomla || {};

Joomla.JMultiSelect = new Class({ <-- ERROR Class is not defined
    initialize : function(table) {
        this.table = document.id(table);
        if (this.table) {
            this.boxes = this.table.getElements('input[type=checkbox]');
            this.boxes.addEvent('click', function(e){
                this.doselect(e);
            }.bind(this));
        }
    },

Answer 1

Browse cache didn't want to clear, I had to open multiselect.js in a separate tab and CTRL+F5 until script refreshed.

Answer 2

If you still having problems refreshing your cache you probably need to replace your legacies js scripts from a fresh joomla install.

So download the exact version you are using, backup your media/system and copy the folder from the joomla install then clean the cache