When to use a javascript constructor and prototypes

In an effort to make my code more testable, I'm trying to make use of constructors and prototypes.

Questions:

1. I'd like to be able to write unit tests for both createAddonMenu and showSidebar, but at this moment I don't believe it's possible as I'm not returning a value from neither method; I'm simply just calling Google's third party code for creating a menu and showing a sidebar. Would I need to create a wrapper around any code calling a third party's code, and then call that within my method?

2. I'd like a sanity check on when I should create a constructor. I feel like maybe I'm trying to shove too many arguments into it, and maybe trying to do too much. Should I create two separate constructors (one for createMenu and one for showSidebar)? Should I even be using constructors at all?

Here is a working Google Apps Script (GAS) for creating a new submenu under the Addon main menu in a Google Spreadsheet, and if a user clicks the 'Show' menu item, a sidebar will appear.

var UI = function (menuName, menuFunction, sidebarFile, sidebarTitle) {
    this.menuName = menuName;
    this.menuFunction = menuFunction;
    this.sidebarFile = sidebarFile;
    this.sidebarTitle = sidebarTitle;
};

UI.prototype.createAddonMenu = function () {
    try {
        SpreadsheetApp.getUi()
            .createAddonMenu()
            .addItem(this.menuName, this.menuFunction)
            .addToUi();
        Logger.log('onOpen (success): building menu');
    } catch (e) {
        Logger.log('onOpen (fail): ' +  e);
    }
};

UI.prototype.showSidebar = function () {
    var ui = HtmlService.createTemplateFromFile(this.sidebarFile)
        .evaluate()
        .setTitle(this.sidebarTitle)
        .setSandboxMode(HtmlService.SandboxMode.IFRAME);

    return SpreadsheetApp.getUi()
        .showSidebar(ui);
};

var ui = new UI('Show', 'showSidebar', 'index', 'Awesome Title');

// onOpen is a built-in Google Apps Script function that gets called when the spreadsheet is opened
function onOpen(e) {
    return ui.createAddonMenu();
}

// showSidebar is a standalone function and returns the ui showSidebar method. needs to be this way in order to be called from createAddonMenu
function showSidebar() {
    return ui.showSidebar();
}