Minimal example of a gnome extension that just shows some text in the menu bar

simple_gnome_extension_icon.js

const St = imports.gi.St;
const Lang = imports.lang;

// UI specific components
const Main = imports.ui.main;
const PanelMenu = imports.ui.panelMenu;

const Indicator = new Lang.Class({
    Name: 'GnomeBitBarIndicator', // does this even matter?
    Extends: PanelMenu.Button,

    _init: function() {
        this.parent(0.0, "GnomeBitBar", false); // does this even matter?
        let text = new St.Label({
            text: "Foo"
        });
        this.actor.add_actor(text);
    }
});

let menu;

function init() {
    // One time startup initialization
}

function enable() {
    menu = new Indicator();
    Main.panel.addToStatusArea('indicator', menu);
}

function disable() {
    menu.destroy();
}