main.js

// @flow
require('dotenv').config()

const electron = require('electron')
const path = require('path')

const { app, BrowserWindow, ipcMain } = electron

let mainWindow

function createWindow() {
  const webPreferences = {
    preload: path.join(__dirname, 'preload.js')
  }

  mainWindow = new BrowserWindow({ webPreferences, width: 1440, height: 900 })

  mainWindow.loadURL('https://google.com')

  mainWindow.on('closed', () => {
    mainWindow = null
  })

  if (process.env.NODE_ENV == "development") {
    mainWindow.webContents.openDevTools()
  }
}

app.on('ready', createWindow)

app.on('window-all-closed', () => {
  if (process.platform !== 'darwin') {
    app.quit()
  }
})

app.on('activate', () => {
  if (mainWindow === null) {
    createWindow()
  }
})

app.on('open-url', () => {
  if (mainWindow) {
    mainWindow.webContents.send('postMessage', 'Send a message via postMessage')
  }
})


ipcMain.on('postMessage', () => {
  console.log('We received a postMessage from the preload script')
})

preload.js

// @flow

const { ipcRenderer } = require('electron')

window.addEventListener('message', ({ data }) => {
  // our embedded site did a window.postMessage and therefor we will
  // proxy it back to our main process
  ipcRenderer.send('postMessage', data)
})

ipcRenderer.on('postMessage', (event, ...args) => {
  // We received an event on the postMessage channel from
  // the main process. Do a window.postMessage to forward it
  window.postMessage([], '*')
})