On-the-fly extend ECB-menu

Say you have business data administrated in a SharePoint list/library, and you want to enable a custom action on the items. In the "old days" one would create a Feature that deploys a CustomAction. But nowadays we intend to / or even must (Office 365 /SharePoint Online) avoid server-side (or sandbox) deployments. Clientside-development is the new mantra. SharePoint as webapplication is strongly javascript-based, from 2007 and beyond. Also the handling of the Edit-Control-Block menu is via SharePoint javascript code. This makes it possible to "break" into that execution, and inject some own handling. Via F12 DeveloperTools I identified where to 'break into'/extend:

var EnrichListView = window.EnrichListView || {};

EnrichListView.UI = function () {

    function MyCustomItemAction(itemId) {
        // do something with the item identified by 'itemId'
    }

    function OverloadMenuHtc_show() {
        $.prototype.base_MenuHtc_show = MenuHtc_show;
         MenuHtc_show = function(oMaster, oParent, fForceRefresh, fFlipTop, yOffset) {
           if ($(oMaster).find("SPAN[id='ID_EditItem']").length == 1) {
               var itemId = oParent.childNodes[0].id;
               var spanInsert = $("<SPAN text='Custom Action on item' 
                    onMenuClick='EnrichListView.UI.MyCustomItemAction(" + itemId + ")' 
                    type='option' 
                    iconAltText sequence='200' 
                    CUICommand='CustomActionItems'
                    ></SPAN>");
               $(oMaster).append(spanInsert);
           }
           $.prototype.base_MenuHtc_show(
               oMaster, oParent, fForceRefresh, fFlipTop, yOffset);
           return false;

        };
    }

    var ModuleInit = (function() {
        ExecuteOrDelayUntilScriptLoaded(OverloadMenuHtc_show, "core.js");
    })();

    // Public interface
    return {
        MyCustomItemAction: MyCustomItemAction
    }
}();

Result:

This client-side approach is on itself future-proof, and also works online. The only caveat is that online Microsoft may change the javascript handling/code on which this code breaks into, without you knowing. The javascript code can also change due an update to SharePoint on-premise installation, but then you're aware of a change in the SharePoint installation and can prepare for the change in advance.