PanelButton

gistfile1.js

const St = imports.gi.St;
const PanelMenu = imports.ui.panelMenu;
const Main = imports.ui.main;

function HelloPanelButton() {
    this._init();
}

HelloPanelButton.prototype = {
    __proto__: PanelMenu.Button.prototype,

    _init: function() {
        PanelMenu.Button.prototype._init.call(this, 0.0);
        let btn = new St.Label({text: 'hello'});
        this.actor.set_child(btn);
    },
};

function main() {
    let hbtn = new HelloPanelButton();
    Main.panel._leftBox.add(hbtn.actor);
}