Requiring electron dialog from render process is undefined

On the Renderer process, you must use the Remote module.

const dialog = require('electron').remote.dialog 

More info:
Electron Dialog API
Electron Remote API

Electron in the newest version have changed the way of requiring the modules. The modules are encapsulated within the electron Name space.

// for getting the electrons modules here the new method now i'm using 1.7.12 Electron version (i don't know what that will be in the future)
// you require electron first! it's a name space (module)
var electron = require("electron");
var remote = electron.remote; // you get all the subModuls directly as property (cool incapsulation)
//remote = require("remote") ===> will not work!!!!

// for dialog it's the same !! but we now use remote as modul
var dialog = remote.dialog;

Also you can use this syntax, to import several modules with less writing and by gathering them all together:

var {remote, ipcRenderer, someOtherModulFromElectron} = electron;

for example in the main.js (main process) we could write such a call:

const electron = require('electron')
const {app, BrowserWindow, Menu} = electron;

in place of :

const electron = require('electron')

// Module to control application life.
const app = electron.app

// Module to create native browser window.
const BrowserWindow = electron.BrowserWindow

//modul for bar menu
const Menu = electron.Menu

I'm surprised none of the answers mention this yet, but since the remote module has been removed and it's preferable not to use the userland remote for a variety of reasons discussed here, you'll want to use ipc to have the main process show the dialog.

renderer.js:

const { ipcRenderer } = require("electron");
ipcRenderer.invoke("showDialog", "message");

main.js:

const { ipcMain, dialog } = require("electron");
ipcMain.handle("showDialog", (e, message) => {
    dialog.showMessageBox(mainWindow, { message });
});