main.js

'use strict';

var app = require('app');
var BrowserWindow = require('browser-window');

var mainWindow = null;

app.on('ready', function() {
    mainWindow = new BrowserWindow({
        height: 600,
        width: 800
    });

    mainWindow.loadUrl('file://' + __dirname + '/app/index.html');
});

Please update it, I had to do:

'use strict';

const {app, BrowserWindow} = require('electron')
let mainWindow = null

app.on('ready', function() {
    mainWindow = new BrowserWindow({
        height: 600,
        width: 800
    });

    mainWindow.loadURL('file://' + __dirname + '/app/index.html');
});

• The require line is wrong.
• It's mainWindow.loadURL, not mainWindow.loadUrl.

I'm using Electron 1.3.4.