MediaWiki:Common.js

/* Any JavaScript here will be loaded for all users on every page load. */ 

function AppendCategoryTreeToSidebar() { 

    try {
        var node = document.getElementById( "p-tb" )
                           .getElementsByTagName('div')[0]
                           .getElementsByTagName('ul')[0];
 
        var aNode = document.createElement( 'a' );
        var liNode = document.createElement( 'li' );
 
        aNode.appendChild( document.createTextNode( 'CategoryTree' ) );
        aNode.setAttribute( 'href' , 'http://vietnamwiki.catglobe.com/Special:CategoryTree' );
        liNode.appendChild( aNode );
        liNode.className = 'plainlinks';
        node.appendChild( liNode );
    } catch(e) {
        // lets just ignore what's happened
        return;
    }

} 

addOnloadHook( AppendCategoryTreeToSidebar );