main.js

const electron = require('electron')
// Module to control application life.
const app = electron.app
// Module to create native browser window.
const BrowserWindow = electron.BrowserWindow
const ipc = require('electron').ipcMain;
const path = require('path')
const url = require('url')

// Keep a global reference of the window object, if you don't, the window will
// be closed automatically when the JavaScript object is garbage collected.
let mainWindow;
var changed;

function createWindow () {
  // Create the browser window.
  mainWindow = new BrowserWindow({width: 800, height: 600})

  // and load the index.html of the app.
  mainWindow.loadURL(url.format({
    pathname: path.join(__dirname, 'index.html'),
    protocol: 'file:',
    slashes: true
  }))

  // Open the DevTools.
  //mainWindow.webContents.openDevTools()

  //onbeforeunload is strange , use window close event in main process instead 
  mainWindow.on('close', function(e){
    if (changed) {
      var choice = electron.dialog.showMessageBox(this,
        {
          type: 'question',
          buttons: ['Yes', 'No'],
          title: 'Confirm',
          noLink: true,
          message: 'File changed, Are you sure you want to quit?'
       });

      if(choice == 1){
       e.preventDefault();
      }
    }
  });
  
  
  // Emitted when the window is closed.
  mainWindow.on('closed', function () {
    // Dereference the window object, usually you would store windows
    // in an array if your app supports multi windows, this is the time
    // when you should delete the corresponding element.
    mainWindow = null
  })

  require('./js/menu')
}

// This method will be called when Electron has finished
// initialization and is ready to create browser windows.
// Some APIs can only be used after this event occurs.
app.on('ready', createWindow)

// Quit when all windows are closed.
app.on('window-all-closed', function () {
  // On OS X it is common for applications and their menu bar
  // to stay active until the user quits explicitly with Cmd + Q
  if (process.platform !== 'darwin') {
    app.quit()
  }
})


app.on('activate', function () {
  // On OS X it's common to re-create a window in the app when the
  // dock icon is clicked and there are no other windows open.
  if (mainWindow === null) {
    createWindow()
  }
})


// In this file you can include the rest of your app's specific main process
// code. You can also put them in separate files and require them here.
ipc.on('setChanged', function(event, para) {
  changed = para;
});