MediaWiki Developer's Handbook/Add JavaScript/Adding a purge tab

Code

function purge() {//Here, we declare the function and it's parameters
        var hist; var url;//Here, we declare two variables
        if (!(hist = document.getElementById('ca-history') )) return;   //Try to set hist as the document's element
                                                                        //with id 'ca-history' - tha's the history tab
        if (!(url = hist.getElementsByTagName('a')[0] )) return;        //Try to set url as the history tab's first
                                                                        //element with the tag 'a'
        if (!(url = url.href )) return; //Try to set url as the href of that element from line 3
        addPortletLink('p-cactions',    //add a PortletLink to the p-cactions area (the tabs at the top of the page)
                        url.replace(/([?&]action=)history([&#]|$)/, '$1purge$2'),       //replace the ?action=history
                                                                                        //part of the href we got from
                                                                                        //the history tab with ?action=purge,
                                                                                        //and set it as the destination
                        'purge',                                //set the text that will appear on the tab
                        'ca-purge',                             //set the internal id for the tab - this is useful for
                                                                //referring to it, and positioning tabs relative to one another
                        'Purge server cache for this page',     //set the mouseover text
                        '0');                                   //the access key
}
if ( wgCanonicalNamespace != "Special" ) addOnloadHook(purge);  //if it's not a special page then hook onto the page loading
                                                                //and run the function purge